Dynamic range optimization in a compressive imaging system

ABSTRACT

A compressive imaging system for optimizing dynamic range during the acquisition of compressed images. A light modulator modulates incident light with spatial patterns to produced modulated light. A light sensing device generates an electrical signal representing intensity of the modulated light over time. The system amplifies a difference between the electrical signal and an adjustable baseline voltage and captures samples of the amplified signal. The adjustable baseline voltage is set to be approximately equal to the average value of the electrical signal. A compressive imaging system for identifying and correcting hot spot(s) in the incident light field. Search patterns are sent to the light modulator and the corresponding samples of the electrical signal are analyzed. Once the hot spot is located, the light modulating elements corresponding to the hot spot may be turned off or their duty cycle may be reduced.

RELATED APPLICATION DATA

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/372,826, filed on Aug. 11, 2010, entitled“Compressive Sensing Systems and Methods”, invented by Richard Baraniuk,Gary Woods, Kevin Kelly, Robert Bridge, Sujoy Chatterjee and LenoreMcMackin, which is hereby incorporated by reference in its entirety asthough fully and completely set forth herein.

FIELD OF THE INVENTION

The present invention relates to the field of compressive sensing, andmore particularly to systems and methods for determining the variationof background light level during the capture of image/video informationusing compressive sensing hardware.

DESCRIPTION OF THE RELATED ART

According to Nyquist theory, a signal x(t) whose signal energy issupported on the frequency interval [−B,B] may be reconstructed fromsamples {x(nT)} of the signal x(t), provided the rate f_(S)=1/T_(S) atwhich the samples are captured is sufficiently high, i.e., provided thatf_(S) is greater than 2B. Similarly, for a signal whose signal energy issupported on the frequency interval [A,B], the signal may bereconstructed from samples captured with sample rate greater than B-A. Afundamental problem with any attempt to capture a signal x(t) accordingto Nyquist theory is the large number of samples that are generated,especially when B (or B-A) is large. The large number of samples istaxing on memory resources and on the capacity of transmission channels.

Nyquist theory is not limited to functions of time. Indeed, Nyquisttheory applies more generally to any function of one or more realvariables. For example, Nyquist theory applies to functions of twospatial variables such as images, to functions of time and two spatialvariables such as video, and to the functions used in multispectralimaging, hyperspectral imaging, medical imaging and a wide variety ofother applications. In the case of an image I(x,y) that depends onspatial variables x and y, the image may be reconstructed from samplesof the image, provided the samples are captured with sufficiently highspatial density. For example, given samples {I(nΔx,mΔy)} captured alonga rectangular grid, the horizontal and vertical densities 1/Δx and 1/Δyshould be respectively greater than 2B_(x) and 2B_(y), where B_(x) andB_(y) are the highest x and y spatial frequencies occurring in the imageI(x,y). The same problem of overwhelming data volume is experienced whenattempting to capture an image according to Nyquist theory. The modemtheory of compressive sensing is directed to such problems.

Compressive sensing relies on the observation that many signals (e.g.,images or video sequences) of practical interest are not onlyband-limited but also sparse or approximately sparse when representedusing an appropriate choice of transformation, for example, atransformation such as a Fourier transform, a wavelet transform or adiscrete cosine transform (DCT). A signal vector v is said to beK-sparse with respect to a given transformation T when thetransformation of the signal vector, Tv, has no more than K non-zerocoefficients. A signal vector v is said to be sparse with respect to agiven transformation T when it is K-sparse with respect to thattransformation for some integer K much smaller than the number L ofcomponents in the transformation vector Tv.

A signal vector v is said to be approximately K-sparse with respect to agiven transformation T when the coefficients of the transformationvector, Tv, are dominated by the K largest coefficients (i.e., largestin the sense of magnitude or absolute value). In other words, if the Klargest coefficients account for a high percentage of the energy in theentire set of coefficients, then the signal vector v is approximatelyK-sparse with respect to transformation T. A signal vector v is said tobe approximately sparse with respect to a given transformation T when itis approximately K-sparse with respect to the transformation T for someinteger K much less than the number L of components in thetransformation vector Tv.

Given a sensing device that captures images with N samples per image andin conformity to the Nyquist condition on spatial rates, it is often thecase that there exists some transformation and some integer K very muchsmaller than N such that the transform of each captured image will beapproximately K sparse. The set of K dominant coefficients may vary fromone image to the next. Furthermore, the value of K and the selection ofthe transformation may vary from one context (e.g., imaging application)to the next. Examples of typical transforms that might work in differentcontexts are the Fourier transform, the wavelet transform, the DCT, theGabor transform, etc.

Compressive sensing specifies a way of operating on the N samples of animage so as to generate a much smaller set of samples from which the Nsamples may be reconstructed, given knowledge of the transform underwhich the image is sparse (or approximately sparse). In particular,compressive sensing invites one to think of the N samples as a vector vin an N-dimensional space and to imagine projecting the vector v ontoeach vector in a series of M vectors {R(i)} in the N-dimensional space,where M is larger than K but still much smaller than N. Each projectiongives a corresponding real number s(i), e.g., according to theexpressions(i)=<v,R(i)>,where the notation <v,R(i)> represents the inner product (or dotproduct) of the vector v and the vector R(i). Thus, the series of Mprojections gives a vector U including M real numbers. Compressivesensing theory further prescribes methods for reconstructing (orestimating) the vector v of N samples from the vector U of M realnumbers. For example, according to one method, one should determine thevector x that has the smallest length (in the sense of the L₁ norm)subject to the condition that ΦTx=U, where Φ is a matrix whose rows arethe transposes of the vectors R(i), where T is the transformation underwhich the image is K sparse or approximately K sparse.

Compressive sensing is important because, among other reasons, it allowsreconstruction of an image based on M measurements instead of the muchlarger number of measurements N recommended by Nyquist theory. Thus, forexample, a compressive sensing camera would be able to capture asignificantly larger number of images for a given size of image store,and/or, transmit a significantly larger number of images per unit timethrough a communication channel of given capacity.

As mentioned above, compressive sensing operates by projecting the imagevector v onto a series of M vectors. As discussed in U.S. patentapplication Ser. No. 11/379,688 (published as 2006/0239336 and inventedby Baraniuk et al.) and illustrated in FIG. 1, an imaging device (e.g.,camera) may be configured to take advantage of the compressive sensingparadigm by using a digital micromirror device (DMD) 40. An incidentlightfield 10 passes through a lens 20 and then interacts with the DMD40. The DMD includes a two-dimensional array of micromirrors, each ofwhich is configured to independently and controllably switch between twoorientation states. Each micromirror reflects a corresponding portion ofthe incident light field based on its instantaneous orientation. Anymicromirrors in a first of the two orientation states will reflect theircorresponding light portions so that they pass through lens 50. Anymicromirrors in a second of the two orientation states will reflecttheir corresponding light portions away from lens 50. Lens 50 serves toconcentrate the light portions from the micromirrors in the firstorientation state onto a photodiode (or photodetector) situated atlocation 60. Thus, the photodiode generates a signal whose amplitude atany given time represents a sum of the intensities of the light portionsfrom the micromirrors in the first orientation state.

The compressive sensing is implemented by driving the orientations ofthe micromirrors through a series of spatial patterns. Each spatialpattern specifies an orientation state for each of the micromirrors. Theoutput signal of the photodiode is digitized by an A/D converter 70. Inthis fashion, the imaging device is able to capture a series ofmeasurements {s(i)} that represent inner products (dot products) betweenthe incident light field and the series of spatial patterns withoutfirst acquiring the incident light field as a pixelized digital image.The incident light field corresponds to the vector v of the discussionabove, and the spatial patterns correspond to the vectors R(i) of thediscussion above.

The incident light field may be modeled by a function I(x,y,t) of twospatial variables and time. Assuming for the sake of discussion that theDMD comprises a rectangular array. The DMD implements a spatialmodulation of the incident light field so that the light field leavingthe DMD in the direction of the lens 50 might be modeled by{I(nΔx,mΔy,t)*M(n,m,t)}

where m and n are integer indices, where I(nΔx,mΔy,t) represents theportion of the light field that is incident upon that (n,m)^(th) mirrorof the DMD at time t. The function M(n,m,t) represents the orientationof the (n,m)^(th) mirror of the DMD at time t. At sampling times, thefunction M(n,m,t) equals one or zero, depending on the state of thedigital control signal that controls the (n,m)^(th) mirror. Thecondition M(n,m,t)=1 corresponds to the orientation state that reflectsonto the path that leads to the lens 50. The condition M(n,m,t)=0corresponds to the orientation state that reflects away from the lens50.

The lens 50 concentrates the spatially-modulated light field{I(nΔx,mΔy,t)*M(n,m,t)}

onto a light sensitive surface of the photodiode. Thus, the lens and thephotodiode together implement a spatial summation of the light portionsin the spatially-modulated light field:

${S(t)} = {\sum\limits_{n,m}{{I\left( {{n\;\Delta\; x},{m\;\Delta\; y},t} \right)}{{M\left( {n,m,t} \right)}.}}}$

Signal S(t) may be interpreted as the intensity at time t of theconcentrated spot of light impinging upon the light sensing surface ofthe photodiode. The A/D converter captures measurements of S(t). In thisfashion, the compressive sensing camera optically computes an innerproduct of the incident light field with each spatial pattern imposed onthe mirrors. The multiplication portion of the inner product isimplemented by the mirrors of the DMD. The summation portion of theinner product is implemented by the concentrating action of the lens andalso the integrating action of the photodiode.

In a compressive sensing (CS) device such as that described above, thequality of image reconstruction may suffer if the measurements of signalS(t) are not captured with sufficiently high resolution. Furthermore,intensely bright spots in the incident light field may make it moredifficult to detect the small scale variations in signal S(t) that areneeded to reconstruct the remaining portions of the incident lightfield. Thus, there exists a need for mechanisms capable of increasing(or optimizing) the resolution with which sensor measurements can beobtained in a CS device, and a need for mechanisms capable of decreasing(or eliminating) the negative effects of hot spots in the incident lightfield.

SUMMARY

In one set of embodiments, a system may be configured to include a lightsensing device, an amplifier, an analog-to-digital converter (ADC) and aswitching subsystem.

The light sensing device may be configured to receive a modulated lightstream and to generate an electrical signal that represents intensity ofthe modulated light stream as a function of time. The modulated lightstream is produced by modulating an incident stream of light with asequence of spatial patterns.

The amplifier may be configured to amplify an input signal in order toobtain an amplified signal. The gain of the amplifier is controllable.

The ADC may be configured to sample the amplified signal in order toobtain a sequence of samples.

The switching subsystem may be configured to controllably select one ofthe following to be the input signal to the amplifier: (a) theelectrical signal from the light sensing device, or (b) a differencesignal representing a difference between the electrical signal and anadjustable baseline voltage.

In an initial mode, the system may set the gain of the amplifier to arelatively low value, direct the switching subsystem to select theelectrical signal from the light sensing device as the input to theamplifier, and then estimate the average value of the electrical signalbased on samples captured from the ADC. The average value may be used tocompute a standard deviation of the electrical signal. When the spatialpatterns used by the modulated the incident light stream are random (orpseudo-random), the standard deviation of the electrical signal may beapproximated by AVG/√{square root over (N)}, where AVG is the averagevalue of the electrical signal, and where N is the number of lightmodulating elements in the light modulation unit.

In a signal acquisition mode, the system may set the gain of amplifierto a larger value that is based on the estimated standard deviation, anddirect the switching subsystem to select the difference signal. Theadjustable baseline voltage is equal to (or approximately equal to) theaverage value of the electrical signal. Thus, the difference signal isequal to (or approximately equal to, or roughly equal to) the ACcomponent of the electrical signal. The system may employ variousmechanisms for generating the adjustable baseline voltage. For example,may use an digital-to-analog converter to generate the adjustablebaseline voltage. As another example, the system may filter theelectrical signal using a lowpass filter.

In another set of embodiments, a system may be configured to include alight modulation unit, a light sensing device, a processor and memory.

The light modulation unit may be configured to modulate an incidentlight stream, where the light modulation unit includes an array of lightmodulating elements.

The memory stores program instructions, where the program instructions,when executed by the processor, cause the processor to perform a searchto identify a subset of the array of light modulating elements whereintensity of the incident light stream is above an intensity threshold.The search includes supplying a first sequence of spatial patterns tothe light modulation unit so that the light modulation unit modulatesthe incident light stream with a first sequence of spatial patterns andthereby produces a first modulated light stream. The search alsoincludes: receiving first samples of an electrical signal produced bythe light sensing device, where the first samples represent intensity ofthe first modulated light stream as a function of time; and operating onthe first samples to identify said subset.

Various additional embodiments are described in U.S. ProvisionalApplication No. 61/372,826, which is hereby incorporated by reference inits entirety as though fully and completely set forth herein.

For information on how to determine and remove variation in backgroundlight level in a compressive imaging system, please refer to thefollowing patent applications, each of which is hereby incorporated byreference in its entirety as though fully and completely set forthherein: (A) U.S. application Ser. No. 13/193,553, filed on Jul. 28,2011, invented by Richard Baraniuk, Kevin Kelly, Robert Bridge, SujoyChatterjee and Lenore McMackin, titled “Determining Light LevelVariation in Compressive Imaging by Injecting Calibration Patterns intoPattern Sequence”; and (B) U.S. application Ser. No. 13/193,556, filedon Jul. 28, 2011, invented by Richard Baraniuk, Kevin Kelly, RobertBridge, Sujoy Chatterjee and Lenore McMackin, titled “Low-Pass Filteringof Compressive Imaging Measurements to Infer Light Level Variation”.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiments isconsidered in conjunction with the following drawings.

FIG. 1 illustrates a compressive sensing camera according to the priorart.

FIG. 2A illustrates one embodiment of a system 100 that is operable tocapture compressive imaging samples and also samples of background lightlevel. (LMU is an acronym for “light modulation unit”. MLS is an acronymfor “modulated light stream”. LSD is an acronym for “light sensingdevice”.)

FIG. 2B illustrates an embodiment of system 100 that includes aprocessing unit 150.

FIG. 2C illustrates an embodiment of system 100 that includes an opticalsubsystem 105 to focus received light L onto the light modulation unit110.

FIG. 2D illustrates an embodiment of system 100 that includes an opticalsubsystem 117 to direct or focus or concentrate the modulated lightstream MLS onto the light sensing device 130.

FIG. 2E illustrates an embodiment where the optical subsystem 117 isrealized by a lens 117L.

FIG. 2F illustrates an embodiment of system 100 that includes a controlunit that is configured to supply a series of spatial patterns to thelight modulation unit 110.

FIG. 3A illustrates system 200, where the light modulation unit 110 isrealized by a plurality of mirrors (collectively referenced by label110M).

FIG. 3B shows an embodiment of system 200 that includes the processingunit 150.

FIG. 4 shows an embodiment of system 200 that includes the opticalsubsystem 117 to direct or focus or concentrate the modulated lightstream MLS onto the light sensing device 130.

FIG. 5A shows an embodiment of system 200 where the optical subsystem117 is realized by the lens 117L.

FIG. 5B shows an embodiment of system 200 where the optical subsystem117 is realized by a mirror 117M and lens 117L in series.

FIG. 5C shows another embodiment of system 200 that includes a TIR prismpair 107.

FIG. 6 illustrates one embodiment of a system 600 configured tocontrollably couple either a detector voltage or a difference betweenthe detector voltage and an adjustable baseline voltage to the input ofan amplifier 637.

FIG. 7 illustrates an embodiment of a system 600 where processing unit150 controls the state of the switching subsystem.

FIG. 8 illustrates an embodiment of a system 600 where adigital-to-analog converter (DAC) is used to generate the adjustablebaseline voltage.

FIG. 9 illustrates an embodiment of a system 600 where the processingunit 150 supplies the digital input word used by the DAC and the gainused by the amplifier 637.

FIG. 10 illustrates an embodiment of a system 600 where the detectorvoltage and the adjustable baseline voltage are subtracted prior to theswitching subsystem 635.

FIG. 11 illustrates an embodiment of a system 600 where processing unit150 receives the samples acquired by the analog-to-digital (ADC) 640.

FIG. 12 illustrates an embodiment of light sensing device 130 thatincludes a transimpedance amplifier 130A.

FIG. 13 illustrates an embodiment of a system 600 where a lowpass filter633 is used to generate the adjustable baseline voltage.

FIG. 14 illustrates an embodiment of a system 600 where the switch 635couples the minus input of the amplifier 637 to either the terminal B ofthe amplifier 630 or to the output of the DAC.

FIG. 15 illustrates an embodiment of a system 600 where the switch 635couples the plus input of the amplifier 637 to either the terminal A ofthe amplifier 630 or to the output of the summing node.

FIG. 16 illustrates an embodiment of a system 600 where the switch 635couples the minus input of the amplifier 637 to the output of thelowpass filter 633 or to the terminal B of the amplifier 630.

FIG. 17 illustrates an embodiment of a system 600 where the switch 635couples the plus input of the amplifier 637 to the terminal A of theamplifier 630 or to the output of highpass filter 637.

FIG. 18A illustrates an embodiment of a method 1800 for improving thequality of compressive imaging measurements.

FIG. 18B illustrates an example of a detector signal, the detectorsignal with DC offset removed, and the detector signal with DC offsetremove and with amplification.

FIG. 19 illustrates one embodiment of a method 1900 for identifyinghigh-intensity regions (e.g., hot spots) in an incident light stream.

FIG. 20 illustrates one embodiment of a method 1900 for attenuating orremoving a high-intensity region(s) in an incident light stream.

FIG. 21 illustrates a process of scanning a region R through the arrayof light modulating elements, in order to locate a hot spot, accordingto one embodiment.

FIG. 22 illustrates a binary search process for locating a hot spot inthe incident light stream, according to one embodiment.

FIG. 23A illustrates a process of moving a band pattern through thelight modulating array, according to one embodiment.

FIG. 23B illustrates a maximizing position of the band pattern.

FIG. 23C illustrates a process of moving a region R across the lightmodulating array and within the maximizing band pattern, according toone embodiment.

FIG. 24A illustrates one embodiment of a process of rotating a sectorpattern within the light modulating array, in order to locate a hotspot.

FIG. 24B illustrates one embodiment of a process of moving an annularring pattern outward from a center of the light modulating array.

FIG. 25 illustrates one embodiment of a system 2500 for identifyinghigh-intensity regions in the incident light stream.

FIG. 26 illustrates an embodiment of a system 2500 where processor 2520receives the samples provided by the ADC 140.

FIG. 27 illustrates an embodiment of a system 2500 where control unit120 supplies spatial patterns to the light modulation unit 110.

FIG. 28 illustrates one embodiment of a compressive imaging system 2800including one or more detector channels.

FIG. 29 illustrates one embodiment of a compressive imaging system 2900where separate portions of the modulated light stream MLS are deliveredto respective light sensing devices.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and are herein described in detail. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following patent applications provide teachings regardingcompressive sensing and compressive imaging:

U.S. Provisional Application No. 60/673,364 entitled “Method andApparatus for Optical Image Compression,” filed on Apr. 21, 2005;

U.S. Provisional Application No. 60/679,237 entitled “Method andApparatus for Reconstructing Data from Multiple Sources,” filed on May10, 2005;

U.S. Provisional Application No. 60/729,983 entitled “Random Filters forCompressive Sampling and Reconstruction,” filed on Oct. 25, 2005;

U.S. Provisional Application No. 60/732,374 entitled “Method andApparatus for Compressive Sensing for Analog-to-Information Conversion,”filed on Nov. 1, 2005;

U.S. Provisional Application No. 60/735,616 entitled “Method andApparatus for Distributed Compressed Sensing,” filed on Nov. 10, 2005;

U.S. Provisional Application No. 60/759,394 entitled “Sudocodes:Efficient Compressive Sampling Algorithms for Sparse Signals,” filed onJan. 16, 2006;

U.S. patent application Ser. No. 11/379,688 entitled “Method andApparatus for Compressive Imaging Device”, filed on Apr. 21, 2006; and

U.S. patent application Ser. No. 12/791,171 entitled “Method andApparatus for Compressive Imaging Device”, filed on Jun. 1, 2010.

Terminology

A memory medium is a non-transitory medium configured for the storageand retrieval of information. Examples of memory media include: variouskinds of semiconductor-based memory such as RAM and ROM; various kindsof magnetic media such as magnetic disk, tape, strip and film; variouskinds of optical media such as CD-ROM and DVD-ROM; various media basedon the storage of electrical charge and/or any of a wide variety ofother physical quantities; media fabricated using various lithographictechniques; etc. The term “memory medium” includes within its scope ofmeaning the possibility that a given memory medium might be a union oftwo or more memory media that reside at different locations, e.g., ondifferent chips in a system or on different computers in a network. Amemory medium is typically computer-readable, e.g., is capable of beingread by a computer.

A computer-readable memory medium may be configured so that it storesprogram instructions and/or data, where the program instructions, ifexecuted by a computer system, cause the computer system to perform amethod, e.g., any of a method embodiments described herein, or, anycombination of the method embodiments described herein, or, any subsetof any of the method embodiments described herein, or, any combinationof such subsets.

A computer system is any device (or combination of devices) having atleast one processor that is configured to execute program instructionsstored on a memory medium. Examples of computer systems include personalcomputers (PCs), workstations, laptop computers, tablet computers,mainframe computers, server computers, client computers, network orInternet appliances, hand-held devices, mobile devices, personal digitalassistants (PDAs), computer-based television systems, grid computingsystems, wearable computers, computers implanted in living organisms,computers embedded in head-mounted displays, computers embedded insensors forming a distributed network, etc.

A programmable hardware element (PHE) is a hardware device that includesmultiple programmable function blocks connected via a system ofprogrammable interconnects. Examples of PHEs include FPGAs (FieldProgrammable Gate Arrays), PLDs (Programmable Logic Devices), FPOAs(Field Programmable Object Arrays), and CPLDs (Complex PLDs). Theprogrammable function blocks may range from fine grained (combinatoriallogic or look up tables) to coarse grained (arithmetic logic units orprocessor cores).

As used herein, the term “light” is meant to encompass within its scopeof meaning any electromagnetic radiation whose spectrum lies within thewavelength range [λ_(L), λ_(U)], where the wavelength range includes thevisible spectrum, the ultra-violet (UV) spectrum, infrared (IR) spectrumand the terahertz (THz) spectrum. Thus, for example, visible radiation,or UV radiation, or IR radiation, or THz radiation, or any combinationthereof is “light” as used herein.

In some embodiments, a computer system may be configured to include aprocessor (or a set of processors) and a memory medium, where the memorymedium stores program instructions, where the processor is configured toread and execute the program instructions stored in the memory medium,where the program instructions are executable by the processor toimplement a method, e.g., any of the various method embodimentsdescribed herein, or, any combination of the method embodimentsdescribed herein, or, any subset of any of the method embodimentsdescribed herein, or, any combination of such subsets.

System 100 for Operating on Light

A system 100 for operating on light may be configured as shown in FIG.2A. The system 100 may include a light modulation unit 110, a lightsensing device 130 and an analog-to-digital converter (ADC) 140.

The light modulation unit 110 is configured to modulate a receivedstream of light L with a series of spatial patterns in order to producea modulated light stream (MLS). The spatial patterns of the series maybe applied sequentially to the light stream so that successive timeslices of the light stream are modulated, respectively, with successiveones of the spatial patterns. (The action of sequentially modulating thelight stream L with the spatial patterns imposes the structure of timeslices on the light stream.) The light modulation unit 110 includes aplurality of light modulating elements configured to modulatecorresponding portions of the light stream. Each of the spatial patternsspecifies an amount (or extent or value) of modulation for each of thelight modulating elements. Mathematically, one might think of the lightmodulation unit's action of applying a given spatial pattern asperforming an element-wise multiplication of a light field vector(x_(ij)) representing a time slice of the light stream L by a vector ofscalar modulation values (m_(ij)) to obtain a time slice of themodulated light stream: (m_(ij))*(x_(ij))=(m_(ij)*x_(ij)). The vector(m_(ij)) is specified by the spatial pattern. Each light modulatingelement effectively scales (multiplies) the intensity of itscorresponding stream portion by the corresponding scalar factor.

The light modulation unit 110 may be realized in various ways. In someembodiments, the LMU 110 may be realized by a plurality of mirrors(e.g., micromirrors) whose orientations are independently controllable.In another set of embodiments, the LMU 110 may be realized by an arrayof elements whose transmittances are independently controllable, e.g.,as with an array of LCD shutters. An electrical control signal suppliedto each element controls the extent to which light is able to transmitthrough the element. In yet another set of embodiments, the LMU 110 maybe realized by an array of independently-controllable mechanicalshutters (e.g., micromechanical shutters) that cover an array ofapertures, with the shutters opening and closing in response toelectrical control signals, thereby controlling the flow of lightthrough the corresponding apertures. In yet another set of embodiments,the LMU 110 may be realized by a perforated mechanical plate, with theentire plate moving in response to electrical control signals, therebycontrolling the flow of light through the corresponding perforations. Inyet another set of embodiments, the LMU 110 may be realized by an arrayof transceiver elements, where each element receives and thenretransmits light in a controllable fashion. In yet another set ofembodiments, the LMU 110 may be realized by a grating light valve (GLV)device. In yet another embodiment, the LMU 110 may be realized by aliquid-crystal-on-silicon (LCOS) device.

In some embodiments, the light modulating elements are arranged in anarray, e.g., a two-dimensional array or a one-dimensional array. Any ofvarious array geometries are contemplated. For example, in someembodiments, the array is a square array or rectangular array. Inanother embodiment, the array is hexagonal. In some embodiments, thelight modulating elements are arranged in a spatially random fashion.

Let N denote the number of light modulating elements in the lightmodulation unit 110. In various embodiments, the number N may take awide variety of values. For example, in different sets of embodiments, Nmay be, respectively, in the range [64, 256], in the range [256, 1024],in the range [1024,4096], in the range [2¹²,2¹⁴], in the range[2¹⁴,2¹⁶], in the range [2¹⁶,2¹⁸], in the range [2¹⁸,2²⁰], in the range[2²⁰,2²²], in the range [2²²,2²⁴], in the range [2²⁴,2²⁶], in the rangefrom 2²⁶ to infinity. The particular value used in any given embodimentmay depend on one or more factors specific to the embodiment.

The light sensing device 130 is configured to receive the modulatedlight stream MLS and to generate an analog electrical signal I_(MLS)(t)representing intensity of the modulated light stream as a function oftime.

The light sensing device 130 may include one or more light sensingelements. The term “light sensing element” may be interpreted as meaning“a transducer between a light signal and an electrical signal”. Forexample, a photodiode is a light sensing element. In various otherembodiments, light sensing elements might include devices such asmetal-semiconductor-metal (MSM) photodetectors, phototransistors,phototubes and photomultiplier tubes.

In some embodiments, the light sensing device 130 includes one or moreamplifiers (e.g., transimpedance amplifiers) to amplify the analogelectrical signals generated by the one or more light sensing elements.

The ADC 140 acquires a sequence of samples {I_(MLS)(k)} of the analogelectrical signal I_(MLS)(t). Each of the samples may be interpreted asan inner product between a corresponding time slice of the light streamL and a corresponding one of the spatial patterns. The set of samples{I_(MLS)(k)} comprises an encoded representation, e.g., a compressedrepresentation, of an image (or a video sequence) and may be used toconstruct the image (or video sequence) based on any constructionalgorithm known in the field of compressive sensing. (For video sequenceconstruction, the samples may be partitioned into contiguous subsets,and then the subsets may be processed to construct correspondingimages.)

In some embodiments, the samples {I_(MLS)(k)} may be used for somepurpose other than, or in addition to, image (or video) construction.For example, system 100 (or some other system) may operate on thecompensated samples to perform an inference task, such as detecting thepresence of a signal or object, identifying a signal or an object,classifying a signal or an object, estimating one or more parametersrelating to a signal or an object, tracking a signal or an object, etc.In some embodiments, an object under observation by system 100 may beidentified or classified by virtue of its sample set {I_(MLS)(k)}, orparameters derived from that sample set, being similar to one of acollection of stored sample sets (or parameter sets).

In some embodiments, the light sensing device 130 includes exactly onelight sensing element. (For example, the single light sensing elementmay be a photodiode.) The light sensing element may couple to anamplifier (e.g., a TIA) (e.g., a multi-stage amplifier).

In some embodiments, the light sensing device 130 may include aplurality of light sensing elements (e.g., photodiodes). Each lightsensing element may convert light impinging on its light sensing surfaceinto a corresponding analog electrical signal representing intensity ofthe impinging light as a function of time. In some embodiments, eachlight sensing element may couple to a corresponding amplifier so thatthe analog electrical signal produced by the light sensing element canbe amplified prior to digitization. System 100 may be configured so thateach light sensing element receives, e.g., a corresponding spatialportion (or spectral portion) of the modulated light stream.

In one embodiment, the analog electrical signals produced, respectively,by the light sensing elements may be summed to obtain a sum signal. Thesum signal may then be digitized by the ADC 140 to obtain the sequenceof samples {I_(MLS)(k)}. In another embodiment, the analog electricalsignals may be individually digitized, each with its own ADC, to obtaincorresponding sample sequences. The sample sequences may then be addedto obtain the sequence {I_(MLS)(k)}. In another embodiment, the analogelectrical signals produced by the light sensing elements may be sampledby a smaller number of ADCs than light sensing elements through the useof time multiplexing. For example, in one embodiment, system 100 may beconfigured to sample two or more of the analog electrical signals byswitching the input of an ADC among the outputs of the two or morecorresponding light sensing elements at a sufficiently high rate.

In some embodiments, the light sensing device 130 may include an arrayof light sensing elements. Arrays of any of a wide variety of sizes,configurations and material technologies are contemplated. In oneembodiment, the light sensing device 130 includes a focal plane arraycoupled to a readout integrated circuit. In one embodiment, the lightsensing device 130 may include an array of cells, where each cellincludes a corresponding light sensing element and is configured tointegrate and hold photo-induced charge created by the light sensingelement, and to convert the integrated charge into a corresponding cellvoltage. The light sensing device may also include (or couple to)circuitry configured to sample the cell voltages using one or more ADCs.

In some embodiments, the light sensing device 130 may include aplurality (or array) of light sensing elements, where each light sensingelement is configured to receive a corresponding spatial portion of themodulated light stream, and each spatial portion of the modulated lightstream comes from a corresponding sub-region of the array of lightmodulating elements. (For example, the light sensing device 130 mayinclude a quadrant photodiode, where each quadrant of the photodiode isconfigured to receive modulated light from a corresponding quadrant ofthe array of light modulating elements. As another example, the lightsensing element 130 may include a bi-cell photodiode.) Each lightsensing element generates a corresponding signal representing intensityof the corresponding spatial portion as a function of time. Each signalmay be digitized (e.g., by a corresponding ADC) to obtain acorresponding sequence of samples. Each sequence of samples may beprocessed to recover a corresponding sub-image. The sub-images may bejoined together to form a whole image.

In some embodiments, the light sensing device 130 includes a smallnumber of light sensing elements (e.g., in respective embodiments, one,two, less than 8, less than 16, less the 32, less than 64, less than128, less than 256). Because the light sensing device of theseembodiments includes a small number of light sensing elements (e.g., farless than the typical modern CCD-based or CMOS-based camera), an entityconfiguring any of these embodiments may afford to spend more per lightsensing element to realize features that are beyond the capabilities ofmodern array-based image sensors of large pixel count, e.g., featuressuch as higher sensitivity, extended range of sensitivity, new range(s)of sensitivity, extended dynamic range, higher bandwidth/lower responsetime. Furthermore, because the light sensing device includes a smallnumber of light sensing elements, an entity configuring any of theseembodiments may use newer light sensing technologies (e.g., based on newmaterials or combinations of materials) that are not yet mature enoughto be manufactured into focal plane arrays (FPA) with large pixel count.For example, new detector materials such as super-lattices, quantumdots, carbon nanotubes and graphene can significantly enhance theperformance of IR detectors by reducing detector noise, increasingsensitivity, and/or decreasing detector cooling requirements.

In one embodiment, the light sensing device 130 is a thermo-electricallycooled InGaAs detector. (InGaAs stands for “Indium Gallium Arsenide”.)In other embodiments, the InGaAs detector may be cooled by othermechanisms (e.g., liquid nitrogen or a Sterling engine). In yet otherembodiments, the InGaAs detector may operate without cooling. In yetother embodiments, different detector materials may be used, e.g.,materials such as MCT (mercury-cadmium-telluride), InSb (IndiumAntimonide) and VOx (Vanadium Oxide).

In different embodiments, the light sensing device 130 may be sensitiveto light at different wavelengths or wavelength ranges. In someembodiments, the light sensing device 130 may be sensitive to light overa broad range of wavelengths, e.g., over the entire visible spectrum orover the entire range [λ_(L),λ_(U)] as defined above.

In some embodiments, the light sensing device 130 may include one ormore dual-sandwich photodetectors. A dual sandwich photodetectorincludes two photodiodes stacked (or layered) one on top of the other.

In one embodiment, the light sensing device 130 may include one or moreavalanche photodiodes.

In some embodiments, a filter may be placed in front of the lightsensing device 130 to restrict the modulated light stream to a specificrange of wavelengths or polarization. Thus, the signal I_(MLS)(t)generated by the light sensing device 130 may be representative of theintensity of the restricted light stream. For example, by using a filterthat passes only IR light, the light sensing device may be effectivelyconverted into an IR detector. The sample principle may be applied toeffectively convert the light sensing device into a detector for red orblue or green or UV or any desired wavelength band, or, a detector forlight of a certain polarization.

In some embodiments, system 100 includes a color wheel whose rotation issynchronized with the application of the spatial patterns to the lightmodulation unit. As it rotates, the color wheel cyclically applies anumber of optical bandpass filters to the modulated light stream MLS.Each bandpass filter restricts the modulated light stream to acorresponding sub-band of wavelengths. Thus, the samples captured by theADC 140 will include samples of intensity in each of the sub-bands. Thesamples may be de-multiplexed to form separate sub-band sequences. Eachsub-band sequence may be processed to generate a corresponding sub-bandimage. (As an example, the color wheel may include a red-pass filter, agreen-pass filter and a blue-pass filter to support color imaging.)

In some embodiments, the system 100 may include a memory (or a set ofmemories of one or more kinds).

In some embodiments, system 100 may include a processing unit 150, e.g.,as shown in FIG. 2B. The processing unit 150 may be a digital circuit ora combination of digital circuits. For example, the processing unit maybe a microprocessor (or system of interconnected of microprocessors), aprogrammable hardware element such as a field-programmable gate array(FPGA), an application specific integrated circuit (ASIC), or anycombination such elements. The processing unit 150 may be configured toperform one or more functions such as image/video construction, systemcontrol, user interface, statistical analysis, and one or moreinferences tasks.

The system 100 (e.g., the processing unit 150) may store the samples{I_(MLS)(k)} in a memory, e.g., a memory resident in the system 100 orin some other system.

In one embodiment, processing unit 150 is configured to operate on thesamples {I_(MLS)(k)} to generate the image or video sequence. In thisembodiment, the processing unit 150 may include a microprocessorconfigured to execute software (i.e., program instructions), especiallysoftware for performing an image/video construction algorithm. In oneembodiment, system 100 is configured to transmit the compensated samplesto some other system through a communication channel. (In embodimentswhere the spatial patterns are randomly-generated, system 100 may alsotransmit the random seed(s) used to generate the spatial patterns.) Thatother system may operate on the samples to construct the image/video.System 100 may have one or more interfaces configured for sending (andperhaps also receiving) data through one or more communication channels,e.g., channels such as wireless channels, wired channels, fiber opticchannels, acoustic channels, laser-based channels, etc.

In some embodiments, processing unit 150 is configured to use any of avariety of algorithms and/or any of a variety of transformations toperform image/video construction. System 100 may allow a user to choosea desired algorithm and/or a desired transformation for performing theimage/video construction.

In some embodiments, the system 100 is configured to acquire a set Z_(M)of samples from the ADC 140 so that the sample set Z_(M) corresponds toM of the spatial patterns applied to the light modulation unit 110,where M is a positive integer. The number M is selected so that thesample set Z_(M) is useable to construct an n-pixel image or n-voxelvideo sequence that represents the incident light stream, where n is apositive integer less than or equal to the number N of light modulatingelements in the light modulation unit 110. System 100 may be configuredso that the number M is smaller than n. Thus, system 100 may operate asa compressive sensing device. (The number of “voxels” in a videosequence is the number of images in the video sequence times the numberof pixels per image, or equivalently, the sum of the pixel counts of theimages in the video sequence.)

In various embodiments, the compression ratio M/n may take any of a widevariety of values. For example, in different sets of embodiments, M/nmay be, respectively, in the range [0.9,0.8], in the range [0.8,0.7], inthe range [0.7,0.6], in the range [0.6,0.5], in the range [0.5,0.4], inthe range [0.4,0.3], in the range [0.3,0.2], in the range [0.2,0.1], inthe range [0.1,0.05], in the range [0.05,0.01], in the range[0.001,0.01].

As noted above, the image constructed from the sample subset Z_(M) maybe an n-pixel image with n≦N. The spatial patterns may be designed tosupport a value of n less than N, e.g., by forcing the array of lightmodulating elements to operate at a lower effective resolution than thephysical resolution N. For example, the spatial patterns may be designedto force each 2×2 cell of light modulating elements to act in unison. Atany given time, the modulation state of the four elements in a 2×2 cellwill agree. Thus, the effective resolution of the array of lightmodulating elements is reduced to N/4. This principle generalizes to anycell size, to cells of any shape, and to collections of cells withnon-uniform cell size and/or cell shape. For example, a collection ofcells of size k_(H)×k_(v), where k_(H) and k_(v) are positive integers,would give an effective resolution equal to N/(k_(H)k_(v)). In onealternative embodiment, cells near the center of the array may havesmaller sizes than cells near the periphery of the array.

Another way the spatial patterns may be arranged to support theconstruction of an n-pixel image with n less than N is to allow thespatial patterns to vary only within a subset of the array of lightmodulating elements. In this mode of operation, the spatial patterns arenull (take the value zero) outside the subset. (Control unit 120 may beconfigured to implement this restriction of the spatial patterns.) Thus,light modulating elements corresponding to positions outside of thesubset do not send any light (or send only the minimum amount of lightattainable) to the light sensing device. Thus, the constructed image isrestricted to the subset. In some embodiments, each spatial pattern(e.g., of a measurement pattern sequence) may be multiplied element-wiseby a binary mask that takes the one value only in the allowed subset,and the resulting product pattern may be supplied to the lightmodulation unit. In some embodiments, the subset is a contiguous regionof the array of light modulating elements, e.g., a rectangle or acircular disk or a hexagon. In some embodiments, the size and/orposition of the region may vary (e.g., dynamically). The position of theregion may vary in order to track a moving object. The size of theregion may vary to dynamically control the rate of image acquisition.

In one embodiment, system 100 may include a light transmitter configuredto generate a light beam (e.g., a laser beam), to modulate the lightbeam with a data signal and to transmit the modulated light beam intospace or onto an optical fiber. System 100 may also include a lightreceiver configured to receive a modulated light beam from space or froman optical fiber, and to recover a data stream from the receivedmodulated light beam.

In one embodiment, system 100 may be configured as a low-cost sensorsystem having minimal processing resources, e.g., processing resourcesinsufficient to perform image (or video) construction in user-acceptabletime. In this embodiment, the system 100 may store and/or transmit thesamples {I_(MLS)(k)} so that another agent, more plentifully endowedwith processing resources, may perform the image/video constructionbased on the samples.

In some embodiments, system 100 may include an optical subsystem 105that is configured to modify or condition the light stream L before itarrives at the light modulation unit 110, e.g., as shown in FIG. 2C. Forexample, the optical subsystem 105 may be configured to receive thelight stream L from the environment and to focus the light stream onto amodulating plane of the light modulation unit 110. The optical subsystem105 may include a camera lens (or a set of lenses). The lens (or set oflenses) may be adjustable to accommodate a range of distances toexternal objects being imaged/sensed/captured.

In some embodiments, system 100 may include an optical subsystem 117 todirect the modulated light stream MLS onto a light sensing surface (orsurfaces) of the light sensing device 130.

In some embodiments, the optical subsystem 117 may include one or morelenses, and/or, one or more mirrors.

In some embodiments, the optical subsystem 117 is configured to focusthe modulated light stream onto the light sensing surface (or surfaces).The term “focus” implies an attempt to achieve the condition that rays(photons) diverging from a point on an object plane converge to a point(or an acceptably small spot) on an image plane. The term “focus” alsotypically implies continuity between the object plane point and theimage plane point (or image plane spot)—points close together on theobject plane map respectively to points (or spots) close together on theimage plane. In at least some of the system embodiments that include anarray of light sensing elements, it may be desirable for the modulatedlight stream MLS to be focused onto the light sensing array so thatthere is continuity between points on the light modulation unit LMU andpoints (or spots) on the light sensing array.

In some embodiments, the optical subsystem 117 may be configured todirect the modulated light stream MLS onto the light sensing surface (orsurfaces) of the light sensing device 130 in a non-focusing fashion. Forexample, in a system embodiment that includes only one photodiode, itmay not be so important to achieve the “in focus” condition at the lightsensing surface of the photodiode since positional information ofphotons arriving at that light sensing surface will be immediately lost.

In one embodiment, the optical subsystem 117 may be configured toreceive the modulated light stream and to concentrate the modulatedlight stream into an area (e.g., a small area) on a light sensingsurface of the light sensing device 130. Thus, the diameter of themodulated light stream may be reduced (possibly, radically reduced) inits transit from the optical subsystem 117 to the light sensing surface(or surfaces) of the light sensing device 130. For example, in someembodiments, the diameter may be reduced by a factor of more than 1.5to 1. In other embodiments, the diameter may be reduced by a factor ofmore than 2 to 1. In yet other embodiments, the diameter may be reducedby a factor of more than 10 to 1. In yet other embodiments, the diametermay be reduced by factor of more than 100 to 1. In yet otherembodiments, the diameter may be reduced by factor of more than 400to 1. In one embodiment, the diameter is reduced so that the modulatedlight stream is concentrated onto the light sensing surface of a singlelight sensing element (e.g., a single photodiode).

In some embodiments, this feature of concentrating the modulated lightstream onto the light sensing surface (or surfaces) of the light sensingdevice allows the light sensing device to sense, at any given time, thesum (or surface integral) of the intensities of the modulated lightportions within the modulated light stream. (Each time slice of themodulated light stream comprises a spatial ensemble of modulated lightportions due to the modulation unit's action of applying thecorresponding spatial pattern to the light stream.)

In some embodiments, the modulated light stream MLS may be directed ontothe light sensing surface of the light sensing device 130 withoutconcentration, i.e., without decrease in diameter of the modulated lightstream, e.g., by use of photodiode having a large light sensing surface,large enough to contain the cross section of the modulated light streamwithout the modulated light stream being concentrated.

In some embodiments, the optical subsystem 117 may include one or morelenses. FIG. 2E shows an embodiment where optical subsystem 117 isrealized by a lens 117L, e.g., a biconvex lens or a condenser lens.

In some embodiments, the optical subsystem 117 may include one or moremirrors. In one embodiment, the optical subsystem 117 includes aparabolic mirror (or spherical mirror) to concentrate the modulatedlight stream onto a neighborhood (e.g., a small neighborhood) of theparabolic focal point. In this embodiment, the light sensing surface ofthe light sensing device may be positioned at the focal point.

In some embodiments, system 100 may include an optical mechanism (e.g.,an optical mechanism including one or more prisms and/or one or morediffraction gratings) for splitting or separating the modulated lightstream MLS into two or more separate streams (perhaps numerous streams),where each of the streams is confined to a different wavelength range.The separate streams may each be sensed by a separate light sensingdevice. (In some embodiments, the number of wavelength ranges may be,e.g., greater than 8, or greater than 16, or greater than 64, or greaterthan 256, or greater than 1024.) Furthermore, each separate stream maybe directed (e.g., focused or concentrated) onto the corresponding lightsensing device as described above in connection with optical subsystem117. The samples captured from each light sensing device may be used toconstruct a corresponding image for the corresponding wavelength range.In one embodiment, the modulated light stream is separated into red,green and blue streams to support color (R,G,B) measurements. In anotherembodiment, the modulated light stream may be separated into IR, red,green, blue and UV streams to support five-channel multi-spectralimaging: (IR, R, G, B, UV). In some embodiments, the modulated lightstream may be separated into a number of sub-bands (e.g., adjacentsub-bands) within the IR band to support multi-spectral orhyper-spectral IR imaging. In some embodiments, the number of IRsub-bands may be, e.g., greater than 8, or greater than 16, or greaterthan 64, or greater than 256, or greater than 1024. In some embodiments,the modulated light stream may experience two or more stages of spectralseparation. For example, in a first stage the modulated light stream maybe separated into an IR stream confined to the IR band and one or moreadditional streams confined to other bands. In a second stage, the IRstream may be separated into a number of sub-bands (e.g., numeroussub-bands) (e.g., adjacent sub-bands) within the IR band to supportmultispectral or hyper-spectral IR imaging.

In some embodiments, system 100 may include an optical mechanism (e.g.,a mechanism including one or more beam splitters) for splitting orseparating the modulated light stream MLS into two or more separatestreams, e.g., where each of the streams have the same (or approximatelythe same) spectral characteristics or wavelength range. The separatestreams may then pass through respective bandpass filters to obtaincorresponding modified streams, wherein each modified stream isrestricted to a corresponding band of wavelengths. Each of the modifiedstreams may be sensed by a separate light sensing device. (In someembodiments, the number of wavelength bands may be, e.g., greater than8, or greater than 16, or greater than 64, or greater than 256, orgreater than 1024.) Furthermore, each of the modified streams may bedirected (e.g., focused or concentrated) onto the corresponding lightsensing device as described above in connection with optical subsystem117. The samples captured from each light sensing device may be used toconstruct a corresponding image for the corresponding wavelength band.In one embodiment, the modulated light stream is separated into threestreams which are then filtered, respectively, with a red-pass filter, agreen-pass filter and a blue-pass filter. The resulting red, green andblue streams are then respectively detected by three light sensingdevices to support color (R,G,B) acquisition. In another similarembodiment, five streams are generated, filtered with five respectivefilters, and then measured with five respective light sensing devices tosupport (IR, R, G, B, UV) multi-spectral acquisition. In yet anotherembodiment, the modulated light stream of a given band may be separatedinto a number of (e.g., numerous) sub-bands to support multi-spectral orhyper-spectral imaging.

In some embodiments, system 100 may include an optical mechanism forsplitting or separating the modulated light stream MLS into two or moreseparate streams. The separate streams may be directed to (e.g.,concentrated onto) respective light sensing devices. The light sensingdevices may be configured to be sensitive in different wavelengthranges, e.g., by virtue of their different material properties. Samplescaptured from each light sensing device may be used to construct acorresponding image for the corresponding wavelength range.

In some embodiments, system 100 may include a control unit 120configured to supply the spatial patterns to the light modulation unit110, as shown in FIG. 2F. The control unit may itself generate thepatterns or may receive the patterns from some other agent. The controlunit 120 and the light sensing device 130 may be controlled by a commonclock signal so that the light sensing device 130 can coordinate(synchronize) its action of capturing the samples {I_(MLS)(k)} with thecontrol unit's action of supplying spatial patterns to the lightmodulation unit 110. (System 100 may include clock generationcircuitry.)

In some embodiment, the control unit 120 may supply the spatial patternsto the light modulation unit in a periodic fashion.

The control unit 120 may be a digital circuit or a combination ofdigital circuits. For example, the control unit may include amicroprocessor (or system of interconnected of microprocessors), aprogrammable hardware element such as a field-programmable gate array(FPGA), an application specific integrated circuit (ASIC), or anycombination such elements.

In some embodiments, the control unit 120 may include a random numbergenerator (RNG) or a set of random number generators to generate thespatial patterns or some subset of the spatial patterns.

In some embodiments, system 100 is battery powered. In some embodiments,the system 100 includes a set of one or more solar cells and associatedcircuitry to derive power from sunlight.

In some embodiments, system 100 includes its own light source forilluminating the environment or a target portion of the environment.

In some embodiments, system 100 may include a display (or an interfaceconfigured for coupling to a display) for displaying constructedimages/videos.

In some embodiments, system 100 may include one or more input devices(and/or, one or more interfaces for input devices), e.g., anycombination or subset of the following devices: a set of buttons and/orknobs, a keyboard, a keypad, a mouse, a touch-sensitive pad such as atrackpad, a touch-sensitive display screen, one or more microphones, oneor more temperature sensors, one or more chemical sensors, one or morepressure sensors, one or more accelerometers, one or more orientationsensors (e.g., a three-axis gyroscopic sensor), one or more proximitysensors, one or more antennas, etc.

Regarding the spatial patterns that are used to modulate the lightstream L, it should be understood that there are a wide variety ofpossibilities. In some embodiments, the control unit 120 may beprogrammable so that any desired set of spatial patterns may be used.

In some embodiments, the spatial patterns are binary valued. Such anembodiment may be used, e.g., when the light modulating elements aretwo-state devices. In some embodiments, the spatial patterns are n-statevalued, where each element of each pattern takes one of n states, wheren is an integer greater than two. (Such an embodiment may be used, e.g.,when the light modulating elements are each able to achieve n or moremodulation states). In some embodiments, the spatial patterns are realvalued, e.g., when each of the light modulating elements admits acontinuous range of modulation. (It is noted that even a two-statemodulating element may be made to effectively apply a continuous rangeof modulation by duty cycling the two states during modulationintervals.)

The spatial patterns may belong to a set of measurement vectors that isincoherent with a set of vectors in which the image/video isapproximately sparse (“the sparsity vector set”). (See “Sparse SignalDetection from Incoherent Projections”, Proc. Int. Conf. Acoustics,Speech Signal Processing—ICASSP, May 2006, Duarte et al.) Given two setsof vectors A={a_(i)} and B={b_(i)} in the same N-dimensional space, Aand B are said to be incoherent if their coherence measure μ(A,B) issufficiently small. The coherence measure is defined as:

${\mu\left( {A,B} \right)} = {\max\limits_{i,j}{{\left\langle {a_{i},b_{j}} \right\rangle }.}}$

The number of compressive sensing measurements (i.e., samples of thesequence {I_(MLS)(k)} needed to construct an N-pixel image (or N-voxelvideo sequence) that accurately represents the scene being captured is astrictly increasing function of the coherence between the measurementvector set and the sparsity vector set. Thus, better compression can beachieved with smaller values of the coherence.

In some embodiments, the measurement vector set may be based on a code.Any of various codes from information theory may be used, e.g., codessuch as exponentiated Kerdock codes, exponentiated Delsarte-Goethalscodes, run-length limited codes, LDPC codes, Reed Solomon codes and ReedMuller codes.

In some embodiments, the measurement vector set corresponds to apermuted basis such as a permuted DCT basis or a permuted Walsh-Hadamardbasis, etc.

In some embodiments, the spatial patterns may be random or pseudo-randompatterns, e.g., generated according to a random number generation (RNG)algorithm using one or more seeds. In some embodiments, the elements ofeach pattern are generated by a series of Bernoulli trials, where eachtrial has a probability p of giving the value one and probability 1−p ofgiving the value zero. (For example, in one embodiment p=½.) In someembodiments, the elements of each pattern are generated by a series ofdraws from a Gaussian random variable.)

The system 100 may be configured to operate in a compressive fashion,where the number of the samples {I_(MLS)(k)} captured by the system 100is less than (e.g., much less than) the number of pixels in the image(or video) to be constructed from the samples. In many applications,this compressive realization is very desirable because it saves on powerconsumption, memory utilization and transmission bandwidth consumption.However, non-compressive realizations are contemplated as well.

In some embodiments, the system 100 is configured as a camera or imagerthat captures information representing an image (or a series of images)from the external environment, e.g., an image (or a series of images) ofsome external object or scene. The camera system may take differentforms in different applications domains, e.g., domains such as visiblelight photography, infrared photography, ultraviolet photography,high-speed photography, low-light photography, underwater photography,multi-spectral imaging, hyper-spectral imaging, etc. In someembodiments, system 100 is configured to operate in conjunction with (oras part of) another system, e.g., in conjunction with (or as part of) amicroscope, a telescope, a robot, a security system, a surveillancesystem, a fire sensor, a node in a distributed sensor network, etc.

In some embodiments, system 100 is configured as a spectrometer.

In some embodiments, system 100 is configured as a multi-spectral orhyper-spectral imager.

In some embodiments, system 100 may also be configured to operate as aprojector. Thus, system 100 may include a light source, e.g., a lightsource located at or near a focal point of optical subsystem 117. Inprojection mode, the light modulation unit 110 may be supplied with animage (or a video sequence) so that the image (or video sequence) can bedisplayed on a display surface (e.g., screen).

In some embodiments, system 100 includes an interface for communicatingwith a host computer. The host computer may send control informationand/or program code to the system 100 via the interface. Furthermore,the host computer may receive status information and/or compressivesensing measurements from system 100 via the interface.

In one realization 200 of system 100, the light modulation unit 110 maybe realized by a plurality of mirrors, e.g., as shown in FIG. 3A. (Themirrors are collectively indicated by the label 110M.) The mirrors 110Mare configured to receive corresponding portions of the light L receivedfrom the environment, albeit not necessarily directly from theenvironment. (There may be one or more optical elements, e.g., one ormore lenses along the input path to the mirrors 110M.) Each of themirrors is configured to controllably switch between two orientationstates. In addition, each of the mirrors is configured to (a) reflectthe corresponding portion of the light onto a sensing path 115 when themirror is in a first of the two orientation states and (b) reflect thecorresponding portion of the light away from the sensing path when themirror is in a second of the two orientation states.

In some embodiments, the mirrors 110M are arranged in an array, e.g., atwo-dimensional array or a one-dimensional array. Any of various arraygeometries are contemplated. For example, in different embodiments, thearray may be a square array, a rectangular array, a hexagonal array,etc. In some embodiments, the mirrors are arranged in a spatially-randomfashion.

The mirrors 110M may be part of a digital micromirror device (DMD). Forexample, in some embodiments, one of the DMDs manufactured by TexasInstruments may be used.

The control unit 120 may be configured to drive the orientation statesof the mirrors through the series of spatial patterns, where each of thepatterns of the series specifies an orientation state for each of themirrors.

The light sensing device 130 may be configured to receive the lightportions reflected at any given time onto the sensing path 115 by thesubset of mirrors in the first orientation state and to generate ananalog electrical signal representing I_(MLS)(t) representing acumulative intensity of the received light portions as function of time.As the mirrors are driven through the series of spatial patterns, thesubset of mirrors in the first orientation state will vary from onespatial pattern to the next. Thus, the cumulative intensity of lightportions reflected onto the sensing path 115 and arriving at the lightsensing device will vary as a function time. Note that the term“cumulative” is meant to suggest a summation (spatial integration) overthe light portions arriving at the light sensing device at any giventime. This summation may be implemented, at least in part, optically(e.g., by means of a lens and/or mirror that concentrates or focuses thelight portions onto a concentrated area as described above).

System realization 200 may include any subset of the features,embodiments and elements discussed above with respect to system 100. Forexample, system realization 200 may include the optical subsystem 105 tooperate on the incoming light L before it arrives at the mirrors 110M,e.g., as shown in FIG. 3B.

In some embodiments, system realization 200 may include the opticalsubsystem 117 along the sensing path as shown in FIG. 4. The opticalsubsystem 117 receives the light portions reflected onto the sensingpath 115 and directs (e.g., focuses or concentrates) the received lightportions onto a light sensing surface (or surfaces) of the light sensingdevice 130. In one embodiment, the optical subsystem 117 may include alens 117L, e.g., as shown in FIG. 5A.

In some embodiments, the optical subsystem 117 may include one or moremirrors, e.g., a mirror 117M as shown in FIG. 5B. Thus, the sensing pathmay be a bent path having more than one segment. FIG. 5B also shows onepossible embodiment of optical subsystem 105, as a lens 105L.

In some embodiments, there may be one or more optical elementsintervening between the optical subsystem 105 and the mirrors 110M. Forexample, as shown in FIG. 5C, a TIR prism pair 107 may be positionedbetween the optical subsystem 105 and the mirrors 110M. (TIR is anacronym for “total internal reflection”.) Light from optical subsystem105 is transmitted through the TIR prism pair and then interacts withthe mirrors 110M. After having interacted with the mirrors 110M, lightportions from mirrors in the first orientation state are reflected by asecond prism of the pair onto the sensing path 115. Light portions frommirrors in the second orientation state may be reflected away from thesensing path.

Optimizing Dynamic Range in Compressive Imaging Measurements

In one set of embodiments, a system 600 may be configured as shown inFIG. 6. The system 600 may include the light sensing device 130 asvariously described above, and may also include switching subsystem 635,amplifier 637 and analog-to-digital converter 640.

The light sensing device 130 is configured to receive a modulated lightstream MLS and to generate an electrical signal v(t) that representsintensity of the modulated light stream as a function of time, e.g., asvariously described above. The modulated light stream is produced bymodulating an incident stream of light with a sequence of spatialpatterns, e.g., as variously described above.

The amplifier 637 is configured to amplify an input signal x(t) in orderto obtain an amplified signal y(t). The gain of the amplifier iscontrollable so that different gains may be applied at different times.

The analog-to-digital converter (ADC) 640 is configured to sample theamplified signal y(t) in order to obtain a sequence of samples {I(k)}.System 600 and/or ADC 640 may be configured to: (a) acquire the samplesso that they are restricted to the steady state portions of the patternmodulation periods (to avoid the transients that occur at transitionsbetween spatial patterns, and/or, (b) capture two or more samples perspatial pattern and average the two or more samples per spatial patternto reduce noise in the samples. For more, information on features (a)and (b), please see U.S. application Ser. No. 13/207,258, filed on Aug.10, 2011, entitled “Techniques for Removing Noise in a CompressiveImaging Device”, invented by Gary L. Woods and James M. Tidman, which ishereby incorporated by reference in its entirety as though fully andcompletely set forth herein.

The switching subsystem 635 is configured to controllably select one ofthe following to be the input signal to the amplifier: (a) theelectrical signal v(t) from the light sensing device, or (b) adifference signal representing a difference between the electricalsignal v(t) and an adjustable baseline voltage (ABV). The switchingsubsystem 635 may be implemented in any of various ways known in the artof electrical engineering. The switching subsystem may be configured topass the selected signal with minimal distortion. The switchingsubsystem may be a semiconductor-based switch.

In some embodiments, system 600 is configured to store a set Z_(M) ofthe samples {I(k)}, where the set Z_(M) corresponds to M of the spatialpatterns (M measurement patterns) and is captured while the differencesignal is selected as the input to the amplifier 637. The value M isselected so that the sample set Z_(M) is usable to construct an N-pixelimage representing the incident light stream, where N is the number oflight modulating elements (in the light modulation unit) used tomodulate the incident light stream, where M is smaller than N. Thus,system 600 may be interpreted as a compressive sensing device, whichobtains compressed representations of images from the incident lightstream.

The compression ratio M/N may take different values in differentembodiments, or in different imaging contexts, or in different noiseenvironments. In different sets of embodiments, the compression ratioM/N may be, respectively, in the range [0.9,0.8], in the range[0.8,0.7], in the range [0.7,0.6], in the range [0.6,0.5], in the range[0.5,0.4], in the range [0.4,0.3], in the range [0.3,0.2], in the range[0.2,0.1], in the range [0.1,0.05], in the range [0.05,0.01]. In someembodiments, the compression ratio may be a user-controllable parameter,e.g., determined by input received through a user interface.

In some embodiments, the system 600 also includes the processing unit150 as described above. See FIG. 7. The processing unit is configured tocontrol the state of the switching subsystem 635 and to control the gainof the amplifier 637. The processing unit may be configured to directthe switching subsystem to select the difference signal as the inputsignal to the amplifier, e.g., during an image acquisition mode. Theadjustable baseline voltage ABV may be set so that it is approximatelyequal to an average value of the electrical signal v(t). Furthermore,the processing unit may set the gain of the amplifier based on anestimate of a standard deviation of the electrical signal v(t). Forexample, the gain may be set so that a predetermined scalar multiple ofthe standard deviation fills the input voltage range of the ADC (or somepredetermined fraction of the input voltage range).

In some embodiments, the action of modulating the incident light streamis performed by a light modulation unit having N light modulatingelements, e.g., by the light modulation 110 as described above. (See,e.g., FIGS. 2A and 2B.) When the spatial patterns applied to the lightmodulation unit are random or pseudo-random, the estimate of thestandard deviation may be a scalar multiple of 1/√{square root over(N)}. For example, in one embodiment, the standard deviation may beestimated by: SD=AVG/√{square root over (N)}, where AVG is the averagevalue (DC value) of the electrical signal v(t).

In some embodiments, system 600 may include a digital-to-analogconverter (DAC) 650 configured to generate the adjustable baselinevoltage based on a digital input word, e.g., as shown in FIG. 8. Theprocessing unit 150 may be configured to set the digital input word ofthe DAC, e.g., as shown in FIG. 9. The processing unit may set thedigital input word based on an estimate of an average value of theelectrical signal v(t), so that the adjustable baseline voltage ABV isapproximately equal to the average value estimate. In addition, theprocessing unit may be configured to: direct the switching subsystem 635to select the difference signal v(t)−ABV as the input signal to theamplifier; and set the gain of the amplifier 637 based on the standarddeviation of the electrical signal, e.g., so that a predetermined scalarmultiple of the standard deviation fills the input voltage range of theADC 640 (or some predetermined fraction of the input voltage range).

In some embodiments, system 600 may include a sum circuit E configuredto determine the difference signal by summing the electrical signal v(t)and a negative of the adjustable baseline voltage ABV, e.g., as shown inFIG. 10.

In some embodiments, system 600 may be configured to estimate theaverage of the electrical signal v(t), e.g., using the processing unit150 as shown in FIG. 11. To enable the average value estimation, theprocessing unit may set a gain of the amplifier 637 to a value G_(A)(e.g., a value that is at the low end of the gain control range of theamplifier 637), and direct the switching subsystem 635 to select theelectrical signal v(t) as the input signal to the amplifier 637.Furthermore, processing unit 150 may determine the estimate for theaverage value of the electrical signal based on one or more of thesamples of the sequence of samples {I(k)}. In one embodiment, the gainvalue G_(A) is selected so that the output range of the light sensingdevice 130 (or the output range of an amplifier intervening between thelight sensing device 130 and amplifier 637) corresponds to the inputvoltage range of the ADC 640.

In some embodiments, the one or more samples used to estimate theaverage value correspond to one or more calibration patterns included inthe sequence of spatial patterns. The control unit 120 as describedabove may be configured to insert the calibration patterns into thesequence of spatial patterns. (See, e.g., FIG. 2F.)

In some embodiments, the one or more calibration patterns include amaximal light transfer pattern and a minimal light transfer pattern. Themaximal light transfer pattern is a pattern that instructs all the lightmodulating elements of the light modulation unit 110 to take the maximumtransfer state. The maximum transfer state of a light modulating elementis the state in which the ratio of the intensity of the light portionoutputted from the element (onto the path that leads to the lightsensing device 130) to the intensity of the light portion incident uponthe element is maximized, within the range of control supported by theelement. (For example, for an element having controllable transmittance,such as an LCD shutter, the maximum transfer state is the state ofmaximum transparency. As another example, for a reflective element suchas a micromirror in a DMD, the maximum transfer state is the orientationstate that reflects the received light portion onto the light sensingpath 115.) Similarly, the minimal light transfer pattern is a patternthat instructs all the light modulating elements of the light modulationunit to take the minimum transfer state. The minimum transfer state of alight modulating element is the state in which the ratio of theintensity of the light portion outputted from the element (onto the paththat leads to the light sensing device 130) to the intensity of thelight portion incident upon the element is minimized, within the rangeof control supported by the element. (For example, for an element havingcontrollable transmittance, such as an LCD shutter, the minimum transferstate is the state of minimum transparency (maximum opacity). As anotherexample, for a reflective element such as a micromirror in a DMD, theminimum transfer state is the orientation state that reflects thereceived light portion away from the light sensing path 115.)

The average value of electrical signal may be estimated as the averageof V_(MAX) and V_(MIN), where V_(MAX) is the amplitude of a samplecorresponding to the maximal transfer pattern and V_(MIN) is theamplitude of a sample corresponding to the minimal transfer pattern. Inone embodiment, the system 600 may be configured to capture two or moresamples corresponding the maximal transfer pattern, in which case,V_(MAX) may be the average of the amplitudes of the two or more samples.Similarly, the system 600 may capture two or more samples correspondingto the minimal transfer pattern, in which case, V_(MIN) may be theaverage of the amplitudes of those two or more samples.

In some embodiments, the one or more calibration patterns include atleast one 50% transfer pattern. A 50% transfer pattern is a spatialpattern that is configured so that 50% of the power in the incidentlight stream passes on average into the modulated light stream. Forexample, a spatial pattern where half the light modulating elements areON (maximum transfer) and the remaining half are OFF (minimum transfer)is a 50% transfer pattern.

In one embodiment, the average value of the electrical signal may beestimated based on one or more samples corresponding to a single 50%transfer pattern, e.g., based on an average of two or more samplescaptured during the assertion of the 50% transfer pattern. In anotherembodiment, the average value of the electrical signal may be estimatedbased on samples captured during the assertion of two or more 50%transfer patterns.

In some embodiments, the one or more calibration patterns include one ormore checkerboard patterns. A checkerboard pattern is a pattern that isbased on a tiling of the array of light modulating elements with arectangular unit cell. The elements in each tile take the samemodulation state (all ON or all OFF). The tiles immediately to thenorth, south, east and west of the arbitrary tile (relative to the axesof the array) take the opposite modulation state as the arbitrary tile.A maximum-resolution checkerboard pattern is one where the tiles are 1×1tiles, each covering only a single element of the light modulatingarray. A checkerboard pattern of lower resolution uses tiles of sizek_(x)×k_(y) where k=k_(x)×k_(y) is greater than one.

In some embodiments, the sequence of spatial patterns includes two ormore calibration patterns, wherein the two or more calibration patternsare not all identical. For example, the calibration patterns may includea first checkerboard pattern and a second checkerboard pattern, wherethe first and second checkerboard patterns are complementary to eachother: one takes the ON state where the other takes the OFF state, andvice versa.

In some embodiments, the light sensing device 130 includes light sensingelement 130E and a transimpedance amplifier (TIA) 130A, e.g., as shownin FIG. 12. The light sensing element 130E generates an electricalsignal u(t) that represent intensity of the modulated light stream MLSas a function of time. The transimpedance amplifier 130 amplifies thesignal u(t) to produce the electrical signal v(t).

In some embodiments, the spatial patterns used to drive the lightmodulation unit include pseudo-random spatial patterns, e.g., asvariously described above. Each pseudo-random spatial pattern may beviewed as a realization of a random process on the array of lightmodulating elements, where the modulation state of each light modulatingelement is determined by a corresponding random variable. The randomvariables over the array of light modulating elements may be independentand identically distributed, or approximately so. For example, eachspatial pattern may be generated so that each element of the pattern hasprobability ½ of being ON and probability ½ of being OFF. (In otherembodiments, the ON probability may be different from ½.) While thepseudo-random patterns are being applied to the incident light stream,the samples corresponding to the pseudo-random patterns may have astandard deviation V₀/√{square root over (N)}, where V₀ is the meanvalue of the signal, where N is the number of light modulating elementsin the light modulation unit 110.

In some embodiments, system 600 may include a lowpass filter 633, e.g.,as shown in FIG. 13. The lowpass filter 633 is configured to low-passfilter the electrical signal v(t) in the analog domain in order togenerate the adjustable baseline voltage ABV. In particular, the lowpassfilter may be configured so that the adjustable baseline voltagecorresponds to the average value (or DC component) of the electricalsignal v(t). The lowpass filter may be implemented in any of a widevariety of ways known in the field of signal processing.

In one embodiment, the system 600 may be configured as shown in FIG. 14.In addition to amplifier 637, the system of FIG. 14 includes anamplifier 630. (For example, the amplifier might be a transimpedanceamplifier. In some embodiments, the amplifier includes one or morestages). The amplifier 630 generates the voltage signal v(t) thatrepresents intensity of the modulated light stream as a function oftime. The voltage v(t) appears across terminals A and B of the amplifier630. The terminal A is coupled to the plus input of the amplifier 637.Switch 635 is configured to selectively connect the minus input of theamplifier 637 to either terminal B or to the adjustable baseline voltageABV based on a control signal provided by processing unit 150. During aninitial mode, processing unit 150 sets the gain to a relatively lowvalue, directs the switch to couple to terminal B, acquires one or moresamples of the resulting sample sequence {I(k)} (e.g., samplescorresponding to one or more calibration patterns), and estimates theaverage of the signal v(t) based on the acquired samples. The processingunit may then compute a standard deviation of the signal v(t) based onthe estimated average. In an image acquisition mode, the processing unit150 sets the gain of the amplifier 637 to a value based on the computedstandard deviation, sets the digital value of the DAC so that theadjustable baseline voltage ABV is approximately equal to the estimatedaverage value, and directs the switch to couple to the ABV. Theresulting sample sequence {I(k)} is a compressed representation of animage or video sequence carried by the incident light stream.

In another embodiment, system 600 may be configured as shown in FIG. 15.The sum circuit Σ generates an output signal corresponding to thedifference between v(t) and the adjustable baseline voltage ABV:v(t)−ABV. The terminal B is connected to the minus input of theamplifier 637. The switch 635 selectively couples the terminal A or theoutput of the sum circuit Σ to the plus input of the amplifier 637. Inthe initial mode, processing unit 150 directs the switch 635 to coupleto terminal A. In the image acquisition mode, the processing unitdirects the switch to couple to the output of the sum circuit.

In yet another embodiment, system 600 may be configured as shown in FIG.16. The lowpass filter 633 generate an output signal that represents theaverage value of the electrical signal v(t). The terminal A is connectedto the plus input of the amplifier 637. The switch 635 selectivelycouples the terminal B or the output of lowpass filter 633 to the minusinput of the amplifier 637. In the initial mode, processing unit 150directs the switch 635 to couple to terminal B. In the image acquisitionmode, the processing unit directs the switch to couple to the output ofthe lowpass filter 633.

In yet another embodiment, system 600 may be configured as shown in FIG.17. The highpass filter 634 generates an output signal that representsthe AC component of the electrical signal v(t), i.e., represents theelectrical signal v(t) minus its average value. In some embodiments, thehigh pass filter may also be configured to attenuate (or remove) lowfrequency noise such as power line noise and/or light noise due toelectrical lighting from the electrical signal v(t). The terminal B iscoupled to the minus input of the amplifier 637. The switch 635selectively couples either the terminal A or the output of the highpassfilter 634 to the plus input of the amplifier 637. In the initial mode,processing unit 150 directs the switch 635 to couple to terminal A. Inthe image acquisition mode, the processing unit directs the switch tocouple to the output of the highpass filter.

As described above, the sample sequence Z_(M) is usable to construct anN-pixel image. Prior to image construction, the set Z_(M) of samples maybe restored by adding the adjustable baseline voltage (or an estimate ofthe average value of the electrical signal v(t)) to each sample of theset Z_(M), thereby obtaining a restored sample set. The restored sampleset may then be used to perform the image construction algorithm. Theadjustable baseline voltage may be known by measurement. For example,the switching subsystem 635 may include a state in which it couples theadjustable baseline voltage ABV to the input x(t) of the amplifier 637,to support measurement of the adjustable baseline voltage by the ADC640. In embodiments where the adjustable baseline voltage is produced bythe DAC 650, the adjustable baseline voltage may be inferred from thedigital input value supplied to the DAC.

Method 1800

In some embodiments, a method 1800 may involve the actions shown in FIG.18A. The method 1800 may be performed using the system 600, in any ofits various embodiments.

Action 1810 includes amplifying an electrical signal with an initialgain in order to obtain an initial amplified signal, e.g., as variouslydescribed above. The amplification may be performed by an amplifier,e.g., the amplifier 637 described above. The electrical signalrepresents intensity of a modulated light stream as a function of time.The modulated light stream is produced by modulating an incident streamof light with a sequence of spatial patterns, e.g., as variouslydescribed above.

Action 1820 includes acquiring one or more samples of the initialamplified signal, e.g., as variously described above. The one or moresamples may correspond to one or more calibration patterns included inthe sequence of spatial patterns.

Action 1830 includes determining an estimate of an average value of theelectrical signal based on the one or more samples, e.g., as variouslydescribed above.

Action 1840 includes amplifying a difference between the electricalsignal and an adjustable baseline voltage with a second gain in order toobtain a second amplified signal, e.g., as variously described above.(This amplifying action may be performed by the same amplifier as usedfor the amplifying action 1810.) The second gain is larger than theinitial gain. The adjustable baseline voltage may be approximately equalto (or correspond to, or based on, or derived from) the average valueestimate. For example, the adjustable baseline voltage may be arrangedto be within +/−C*AVG/sqrt(N) of the estimated average value, where AVGis the average value estimate, where C is a positive real number lessthan 20. In different embodiments, the number C may take differentvalues. For example, in different sets of embodiments, the number C maybe, respectively, less than or equal to 4, less than or equal to 2, lessthan or equal to 1, less than or equal to ½, less than or equal to ¼,less than or equal to ⅛.

Action 1850 includes acquiring samples of the second amplified signalcorresponding to measurement patterns included in the sequence ofspatial patterns.

The measurement patterns may be configured as variously described above.In some embodiments, the measurement patterns may be pseudo-randompatterns, e.g., as variously described above.

In some embodiments, the samples of the second amplified signal includea set of samples corresponding to M of the measurement patterns, and theset of samples is usable to construct an N-pixel image representing theincident light stream, where M is smaller than N, where N is the numberof light modulating elements used to modulated the incident lightstream. The compression ratio M/N may take different values in differentembodiments, or in different imaging contexts, or in different noiseenvironments, e.g., as variously described above.

In some embodiments, the second gain is based on an estimate of astandard deviation of the electrical signal. The standard deviation maydetermined according the expression AVG/Sqrt(N), where AVG is theestimate of the average value of the electrical signal, where N is thenumber of light modulating element in the light modulation unit thatmodulates the incident light stream.

In some embodiments, the method 1800 also includes low-pass filteringthe electrical signal to generate the adjustable baseline voltage.

In some embodiments, the one or more calibration patterns include amaximal light transfer pattern and a minimal light transfer pattern asdescribed above.

In one embodiment, a method for increasing the dynamic range associatedwith a CS acquisition may include: amplifying an electrical signal witha first gain in order to obtain a first amplified signal, wherein saidamplifying is performed by an amplifier, wherein the electrical signalrepresents intensity of a modulated light stream as a function of time,wherein the modulated light stream is produced by modulating an incidentstream of light with a sequence of spatial patterns; acquiring one ormore samples of the first amplified signal corresponding to one or morecalibration patterns included in the sequence of spatial patterns;determining an estimate of an average value of the electrical signalbased on the one or more samples; amplifying a highpass filtered versionof the electrical signal with a second gain in order to obtain a secondamplified signal, wherein the second gain is larger than the first gain;and acquiring samples of the second amplified signal corresponding tomeasurement patterns included in the sequence of spatial patterns. Thisembodiment may incorporate any subset of the features, method actionsand embodiments described above.

In a compressive sensing device, it is possible for the system, beforeacquiring the sensor measurements that are to be used to reconstruct animage frame, to estimate the range of luminosity to be expected in thosesensor measurements. This estimation can be accomplished by making areasonably small number of preliminary sensor measurements, to determinethe range of expected ADC output values. This information can be used bya processing element to minimize noise by dynamically controlling theperformance of the components in the data path.

The luminosity dynamic range of an image may be described as the rangeof luminosity between the darkest pixel in the image, and the brightestpixel in the image. For example, an image taken outside on a shady dayhas a smaller range of luminosity between the darkest and brightestpixels of the image, compared to an image taken on a sunny day where theimage includes the sun and also objects in deep shadow.

In a conventional digital camera, the image output, in the optimaldynamic range case, assigns the brightest pixel a full scale luminositydigital output, and the darkest pixel a zero scale luminosity digitaloutput. This optimal dynamic range can be achieved by a range scalingADC that includes both a pre-amplifier and an ADC block, and where theADC has its pre-amplifier gain and offset setting adjusted to achievethe ideal output code assignments. With such a range-scaling ADC, thestep size of the ADC is minimized, allowing the smallest possibledifferences in luminosity to be measured for a given number of bits ofthe ADC. A digital camera may have one ADC per row or column, or onlyone for the entire array.

The dynamic range of digital cameras is rather limited. A full well in aCCD camera might be on the order of 50,000 electrons, while the minimumreadout noise (which sets the lower bound of detectable light levels)might be on the order of 5 electrons rms. So the dynamic range of thisCCD camera would be on the order of 1e4, corresponding to about 14 bitsin an ADC, if the ADC gain and offset were suitably set.

A similar effect occurs in film cameras. In this case, the signal is nota digital output from an ADC but the photometric density of grains inthe film. In general, film has much higher dynamic range than digitalcameras but the principle is the same. At low light levels, the image islost in the noise, and at high light levels the image saturates anddetail is lost.

Compressive sensing (CS) devices do not suffer from the same dynamicrange limitations as do standard pixel arrays. In some embodiments, a CSdevice may include a spatial light modulator (with N light modulatingelements) and a single photosensor or a small small number ofphotosensors (e.g., less than 40 photosensors). For example, consider asequence of measurements in the single photosensor case. In someembodiments, the spatial patterns supplied to the light modulation unitduring image/video acquisition are random patterns, with each patternconfigured so that approximately ½ of the light modulating elements willbe ON, yielding a photo-voltage at the detector of value V0. If all thelight modulating elements were on, one would measure 2V0. For a set of Mrandom patterns, we expect the corresponding voltage measurements to beclustered around V0 with standard deviation on the order of V0/sqrt(N)(for perfectly random images). Let us consider a case in which about 1%of the light modulating elements have 100× more intensity than theothers (e.g., the sun appears in 10% of the linear field of view of thedevice). This will cause the average photo voltage to increase by afactor of 2 compared to the uniform case, and the standard deviation ofthe measurements will still be on the order of 2V0/sqrt(N). A pixelarray would have a large section of the image (and maybe all of it)wiped out by the higher illumination, whereas the CS device only needsto accommodate a factor of two increase in photo voltage. So the dynamicrange is in principle much higher than for a pixel array. The practicallimit to the dynamic range comes not from the quantization noise or thegain range of the ADC but the technical noise on the source or just thewhite shot noise or Johnson noise of the detector system.

Minimizing the Impact of ADC-Induced Noise by Adjusting the Analog InputSignal to the ADC

To prevent quantization noise and other ADC noise from dominating thenoise properties of the image, we can quantize the signal with enoughbits so that the ADC noise (ideally 1/sqrt(12) the minimum AD unit rms)is much smaller than the other sources of noise in the system, and is assmall as possible relative to the signal amplitude.

Sources of ADC noise:

1. Quantization noise: An ADC will convert the analog input signal intoa digital output code. For example, an n-bit ADC will have 2^n digitaloutput codes that are used to digitize the full analog input. A numberof closely related analog input values may be converted to the sameoutput code because there are not an infinite number of digital outputcodes. The error caused by the finite number of output codes is calledquantization error. The maximum quantization error is bounded by thefull scale analog input range divided by (2^n)/2.

2. Inherent electrical noise: An ADC will have some output noise even inthe presence of a constant, noise-free analog input signal (for example,shot noise).

ADC noise can be minimized by subtracting off (at least approximately)the DC value of the signal, V0, and sampling the difference signal. FIG.18B shows the original signal (left) and the difference signal (middle).As suggested by the signal graph at the right of the figure, thedifference signal may be amplified (prior to sampling) so that it fillsmore (or most) of the ADC input range.

In some embodiments, the CS device may be configured to generate abaseline voltage near V0, within say +/−V0/(2 sqrt(N)), and then adifferential ADC measurement may be performed between the signal and thebaseline voltage, with the ADC full scale adjusted to on the order ofabout 10-20 times V0/sqrt(N). The baseline voltage could be generated(for example) by a DAC with sufficiently high resolution. To estimatethe DC value V0, a preamble sequence could be applied to the spatiallight modulator to allow measurement of the full scale level (2V0) andthe zero level. Alternatively, a standard exemplary pattern of 50%on/off pixels could be applied and measured. Prior to imagereconstruction, the baseline voltage may be added back in to themeasurement set to obtain a restored measurement set. The imagereconstruction operates on the restored measurement set. The baselinevoltage can be determined from the code provided to the DAC, or could bemeasured by the full-scale ADC that is used to measure the full scale2V0 value.

The method can be modified to account for drift in DC value of sourceillumination. An improvement is to perform a “preamble” pattern sequenceat regular intervals, faster than the expected drift. The preamble wouldmay include a fully-on spatial pattern and a fully-off spatial patternin order to measure the value of V0 (and its offset) accurately. Withevery preamble sequence, a new DC value is generated (essentially a newV0). The measurement would be performed by an ADC set to have full scalerange at least as large as 2V0. Then a DAC could be used (for example)to generate the corresponding DC voltage for subtraction from thesignal. If the preamble is applied often enough, it may be possible toremove much of the full-scale drift and baseline drifts in a CSmeasurement. Full-scale drift is a change of the maximum value (oraverage value V0) seen by the detector due to a change in theillumination level from the source being imaged. For instance if theillumination level on the source being imaged changed by +20%, thefull-scale drift would also be +20%. Baseline drift is a change in theoutput signal level when there is no light incident on the camera. Inthe ideal case, when no light is incident on the camera, the detector'soutput signal level should be zero. However in a real circuit thereusually is a nonzero-signal in this case, which we term the baselinesignal. The baseline signal can arise from various sources, includingdark current in the detector, offset voltage of the amplifier, etc.

Hot Spot Identification and Correction

In one set of embodiments, a method 1900 for identifying one or more hotspots in an incident light stream may include the actions shown in FIG.19. The method 1900 may be performed by any of the various systemsdescribed above, e.g., by system 100 or system realization 200 or system600.

Method 1900 is a method for operating a device including an array oflight modulating elements and a light sensing device, e.g., as variouslydescribed above. The array of light modulating elements is configured tomodulate an incident light stream. Each of the light modulating elementsis configured to modulate a corresponding portion of the incident lightstream, e.g., as variously described above.

Action 1910 includes executing a search algorithm to identify a subsetof the array of light modulating elements where intensity (e.g., averageenergy per unit area) of the incident light stream is above an intensitythreshold. The action of executing the search algorithm may includeoperations 1912-1918 described below. The search algorithm may bedirected (controlled) by the processing unit 150 as described above(e.g., by a microprocessor operating under program control).

Action 1912 includes generating a sequence of spatial patterns {P(k)},or directing some other agent (such as control unit 120) to generate thesequence of spatial patterns {P(k)}.

Action 1914 includes modulating the incident stream of light with thesequence of spatial patterns {P(k)} to produce a modulated light streamMLS₁ (or directing some other agent such as the light modulation unit110 to modulate the incident light stream with the pattern sequence{P(k)}). The action of modulating the incident light stream is performedby the array of light modulating elements.

Action 1916 includes receiving samples {I₁(k)} of an electrical signalproduced by the light sensing device. The samples {I₁(k)} representintensity of the modulated light stream MLS₁ as a function of time.

Action 1918 includes operating on the samples {I₁(k)} to identify thesubset of array elements where the intensity of the incident lightstream is above the intensity threshold. This action of operating on thesamples may be performed by processing unit 150 (e.g., by amicroprocessor under program control).

In some embodiments, the method 1900 also includes actions 1920-1940 asshown in FIG. 20.

Action 1920 includes generating a sequence of spatial patterns {Q(k)}(or directing the control unit 120 to generate the sequence of spatialpatterns {Q(k)}), where each of the spatial patterns of the sequence{Q(k)} specifies that the light modulating elements in the identifiedsubset are to attenuate their respective portions of the incident lightstream. In some embodiments, a sequence of measurement patterns (e.g.,drawn from any of the various measurement vector sets described above)may be modified to obtain a modified sequence of patterns, and themodified sequence may be used as the pattern sequence {Q(k)}. Each ofthe measurement patterns may be modified by attenuating (e.g., scalingby a factor less than one) the values of measurement pattern thatcorrespond to the identified subset. Outside the identified subset, thevalues of the measurement pattern may be unmodified. (Note that the term“attenuate” includes within its scope of meaning the possibility ofcompletely nullifying—setting to the OFF state—the pattern valuescorresponding to the identified subset.)

Action 1930 includes modulating the incident light stream with thesequence of spatial patterns {Q(k)} to produce a modulated light streamMLS₂. The modulating action is performed by the array of lightmodulating elements and implements the attenuation of the incident lightportions that correspond to the identified subset. The attenuation of alight portion of the incident light stream means that the correspondingmodulated light portion that becomes a part of the modulated lightstream MLS₂ is decreased in intensity relative to the incident lightportion.

Action 1940 includes receiving samples {I₂(k)} of the electrical signalproduced by the light sensing device. The samples {I₂(k)} representintensity of the modulated light stream MLS₂ as a function of time. Thesamples {I₂(k)} are usable to construct an image or video sequencerepresenting the incident light stream.

In some embodiments, the action of modulating the incident light streamwith the sequence of spatial patterns {Q(k)} includes attenuating theportions of the incident light stream corresponding to the identifiedsubset by dithering (or duty cycling) the respective light modulatingelements during a period of application of each spatial pattern of thesequence {Q(k)}. A light modulating element may be dithered by changingits modulation state within a pattern modulation period. For example, byforcing the element to spend p percent of the modulation period in theON state and the remaining portion of the modulation period in the OFFstate, the element can effectively multiply the intensity of theincident light portion by the scalar factor p/100.

In some embodiments, each of the spatial patterns of the sequence {Q(k)}specifies that the light modulating elements in the identified subsetare to nullify their respective portions of the incident light stream.(For example, in the case where the light modulation unit is a DMD, eachof the spatial patterns {Q(k)} may be configured to turn OFFmicromirrors within the identified subset.) Thus, the portions of theincident light stream corresponding to the identified subset may beprohibited from entering into the modulated light stream MLS₂.

In some embodiments, the search algorithm of action 1910 includes abinary search algorithm. However, other types of searches may also beused.

In some embodiments, the sequence of spatial patterns {P(k)} includessearch patterns representing a partition of the array of lightmodulating elements into a collection of non-overlapping regions. Eachof the search patterns corresponds to a respective one of the regions.(For example, each of the search patterns may specify the maximumtransfer state for light modulating elements belonging to thecorresponding region, and specify the minimum transfer state for lightmodulating elements outside the corresponding region.) The action ofoperating on the samples {I₁(k)} may include extracting search samplescorresponding to the search patterns from the samples {I₁(k)}, anddetermining one or more of the search samples whose amplitudes exceed avoltage threshold V_(R) (or an energy-per-unit-area threshold).

In some embodiments, the method 1900 also includes injecting refinedsearch patterns into the sequence of spatial patterns {P(k)}, where therefined search patterns are localized to a neighborhood of the one ormore regions that correspond to one or more threshold-exceeding searchsamples. By saying that a refined spatial pattern is localized to aneighborhood in the light modulating array means that it is zero (OFF)outside of the neighborhood. A neighborhood of a set of regions is asubset of the light modulating array that contains the set of regions.

In some embodiments, the voltage threshold V_(R) may be used toimplement the above-mentioned intensity threshold.

In some embodiments, the voltage threshold V_(R) may be programmable.

In some embodiments, the threshold V_(R) may be set to valuerepresenting a predetermined percentage of the full scale voltage outputof the light sensing device 130, e.g., a percentage determined based onthe area of each region and the intensity threshold.

In some embodiments, the threshold V_(R) may be set based on knowledgeof voltage values V_(MAX) and V_(MIN), where V_(MAX) is the voltageproduced by the light sensing device 130 in response to the assertion ofa maximal light transfer pattern by the light modulating array, whereV_(MIN) is the voltage produced by the light sensing device 130 inresponse to the assertion of a minimal light transfer pattern by thelight modulating array. For example, the voltage threshold may be set toV_(R)=V_(MIN)+α(V_(MAX)−V_(MIN)), where α is a positive real number lessthan one. The parameter α may be programmable and/or determined by userinput. In some embodiments, the method 1900 includes inserting themaximal light transfer pattern and the minimal light transfer patterninto the pattern sequence {P(k)}, extracting the corresponding samplesfrom the sample sequence {I₁(k)}, and then computing the threshold V_(R)based on the extracted samples.

In some embodiments, the processing unit 150 may perform a histogram onthe search samples, and determine the voltage threshold based on thehistogram.

In some embodiments, the voltage threshold may be determined based onuser input.

As an alternative to determining the one or more search samples thatexceed a voltage threshold, the method 1900 may involve determining then_(L) largest of the search samples, where n_(L) is a positive integer.In some embodiments, the method 1900 also includes injecting refinedsearch patterns into the pattern sequence {P(k)}, where the refinedsearch patterns are localized to a neighborhood of the n_(L) regionsthat correspond to the n_(L) largest of the search samples.

The integer n_(L) may have different values in different embodiments. Insome embodiments, the value of integer n_(L) may be programmable. Insome embodiments, the value of integer n_(L) may vary dynamically.

In some embodiments, the integer n_(L) may be related to the number ofregions n_(R) in the above-mentioned collection of regions. For example,in different sets of embodiments, the ratio n_(L)/n_(R) may be,respectively, less than or equal to 0.01, less than or equal to 0.05,less than or equal to 0.1, less than or equal to 0.2, less than or equalto 0.3, less than or equal to 0.4, less than or equal to 0.6, less thanor equal to 0.8.

In some embodiments, the current value of the integer n_(L) may be basedon the size of a previously-identified hot spot (intensely bright area)in the incident light stream.

In some embodiments, the regions discussed above are equal-area regions.In other embodiments, the regions may have non-uniform areas, in whichcase the processing unit 150 may normalize the search samples so thatthey represent energy per unit area over the respective regions. Forexample, each search sample may be divided by the area of thecorresponding region to obtain the corresponding normalized sample.

In some embodiments, each of the regions is a contiguous region, e.g., aconvex region such as a rectangle, a triangle or a hexagon.

In some embodiments, the sequence of spatial patterns {P(k)} includessearch patterns, where each of the search patterns corresponds to arespective rectangular subset of the array of light modulating elements.

In some embodiments, the sequence of spatial patterns {P(k)} includes asequence of overlapping angular sector patterns and/or a sequence ofoverlapping concentric annular patterns.

In some embodiments, the sequence of spatial patterns {P(k)} includes asequence of overlapping parallel band patterns, and the action 1918 ofoperating on the samples {I₁(k)} includes identifying a subset of thesamples {I₁(k)} that correspond to the band patterns and selecting oneof the samples from the identified subset of samples whose amplitude ismaximum. (For example, the processing unit 150 may slide an initial bandpattern across the light modulating array in the direction perpendicularto the band's long axis.) After the maximizing sample is selected, asequence of rectangle patterns (e.g. overlapping rectangle patterns) maybe inserted into the sequence of spatial patterns {P(k)}, where therectangular patterns are restricted to the maximizing band pattern(i.e., the band pattern corresponding to the maximizing sample). Thisallows the processing unit 150 to identify where the hot spot is locatedwithin the maximizing band pattern.

In some embodiments, the first sequence of spatial pattern includessearch patterns, where the search patterns include one or more patternsthat correspond to respective translates of a previously-identified hotspot region. Since the position and area of a hot spot are likely tovary continuously in time, the search patterns may be designed to searcha neighborhood of the previously-identified hot spot region. Thus, eachof the search patterns may be a vector-translated version of thepreviously-identified hot spot region, with each pattern using adifferent translation vector.

FIG. 21 shows one embodiment of the search algorithm 1910. The sequenceof spatial patterns {P(k)} is arranged so that a rectangular region Rmoves through the light modulating array 2110 in a raster fashion. Inany given spatial pattern, the light modulating elements in the region Rmay be turned ON, and the light modulating elements outside in theregion R may be turned OFF. The area of the array covered by the hotspot(s) is indicated by large values of sample amplitude, i.e.,amplitude of samples {I₁(k)} from the ADC 140 (or the ADC 640). It isnoted that the presence of single hot spot in FIG. 21 is not meant to belimiting. Indeed, the design principles described herein may be used toidentify a plurality of hot spots. Furthermore, the size of the region Rshown in FIG. 21 relative to the light modulating array is not meant tobe limiting. A wide range of sizes are contemplated.

In some embodiments, the region R is moved so that successive instancesof the region (between on pattern and the next) overlap with each other.

FIG. 22 illustrates one embodiment, where the search algorithm isorganized as a binary search. The spatial pattern A is ON over the lefthalf of the array, and OFF over the right half of the array. The secondspatial pattern B is the complement of pattern A. Spatial pattern C isturned ON only in the upper right quadrant. Spatial pattern D is turnedON only in the lower right quadrant. The incident light stream has a hotspot located as shown in the upper right quadrant. (For example, in avisible-light application, the hot spot might be the sun or thereflection of the sun off a building.) The processing unit 150 may startby directing the light modulating array to apply patterns A and B to theincident light stream. For the image shown, the sample induced bypattern B will be significantly larger than the sample induced bypattern A. The processing unit 150 may then eliminate pattern A anddescend to sub-regions of pattern B. In particular, processing unit 150may direct the light modulating array to apply patterns C and D. Thesample induced by pattern C will be significantly larger than the sampleinduced by pattern D. Thus, the processing unit 150 may eliminatepattern D, and explore within pattern C.

In some embodiments, the binary search may explore along both regionsif, at any given stage, the amplitudes of the corresponding inducedsamples are equal (or approximately equal).

In some embodiments, a binary search may be performed to determine asub-region that is smaller than the hot spot and likely to be in theinterior of the hot spot. Then an additional (non-binary) search may beperformed, starting with the determined sub-region, to more preciselyidentify the boundary of the hot spot, e.g., by a process of growingfrom within the interior.

In some embodiments, the search algorithm may involve sliding a bandpattern across the light modulating array, e.g., in a directionperpendicular to the axis of the band pattern as shown FIG. 23A. Thedisplacement of the band pattern between successive patterns of thesequence {P(k)} may programmable. When the band pattern cuts through thecenter of the hot spot, as suggested in FIG. 23B, the induced sampleamplitude attains a maximum. The band pattern that achieves thiscondition may be referred to as the “maximizing band pattern”. Once themaximizing band pattern has been identified, the processing unit 150 mayinsert patterns into the sequence {P(k)} to effect the sliding movementof a rectangle R within the maximizing band pattern as shown in FIG.23C. This allows the processing unit 150 is to more precisely identifythe area of the hot spot. When the rectangle traverses the boundary ofthe hot spot, the induced samples will exhibit a rapid rise inamplitude. The position of the rectangle that gives the maximum ofinduced sample amplitude may be used as an indicator of the hot spotlocation within the maximizing band.

In some embodiments, the search algorithm may involve inserting asequence of sector patterns into the pattern sequence {P(k)} to effectthe rotation of a sector region around a center position C of the lightmodulating array, e.g., as shown in FIG. 24A. A maximizing value ofsample amplitude may be used as an indicator of the sector in which thehot spot reside. The sample amplitudes may be normalized prior tocomparison/analysis to compensate for unequal sector areas. (A sectorthat intersects a corner of the array may be larger in area that asector that intersects the midpoint of a side of the array.)

In some embodiments, the search algorithm may involve inserting a diskpattern and a sequence of annulus patterns into the pattern sequence{P(k)} to effect the expanding growth of an annulus about a constantcenter position C, e.g., as shown in FIG. 24B.

System 2500

In one set of embodiments, a system 2500 for identifying one or more hotspots in an incident light stream may be configured as shown in FIG. 25.The system 2500 may include the light modulation unit 110 and the lightsensing device 130 as described above, and may also include memory 2510and a processor 2520 (e.g., a set of one or more microprocessors).(Furthermore, system 2500 may include any subset of the features,embodiments and elements described above in connection with system 100,system realization 200, system 600 and method 1900.)

The light modulation unit 110 is configured to modulate an incidentlight stream L, e.g., as variously described above. The light modulationunit includes an array of light modulating elements, e.g., as variouslydescribed above.

Memory 2510 stores program instructions. The program instructions, whenexecuted by the processor 2520, cause the processor to perform a searchto identify a subset of the array of light modulating elements whereintensity of the incident light stream is above an intensity threshold,e.g., as variously described above. The search includes: (a) supplying afirst sequence {P(k)} of spatial patterns to the light modulation unit110 so that the light modulation unit modulates the incident lightstream with the first sequence of spatial patterns and thereby producesa first modulated light stream MLS₁; (b) receiving a first sequence ofsamples {I₁(k)} of an electrical signal v(t) produced by the lightsensing device 130, where the first sequence of samples {I₁(k)}represents intensity of the first modulated light stream as a functionof time; and (c) operating on the first sequence of samples {I₁(k)} toidentify the subset.

In some embodiments, the program instructions, when executed by theprocessor 2520, further cause the processor 2520 to: (d) supply a secondsequence {Q(k)} of spatial patterns to the light modulation unit 110 sothat the light modulation unit modulates the incident light stream withthe second sequence of spatial patterns and thereby produces a secondmodulated light stream MLS₂, where each of the spatial patterns of thesecond sequence {Q(k)} specifies that the light modulating elements inthe identified subset are to attenuate their respective portions of theincident light stream; and (e) receive a second sequence of samples{I₂(k)} of the electrical signal v(t) produced by the light sensingdevice 130, where the second sequence of samples represents intensity ofthe second modulated light stream MLS₂ as a function of time. The secondsequence of samples {I₂(k)} is usable to construct an image or videosequence representing the incident light stream, e.g., as variouslydescribed above.

In some embodiments, each spatial pattern of the second sequence {Q(k)}specifies that the light modulation unit 110 is to attenuate theportions of the incident light stream corresponding to the identifiedsubset by dithering the respective light modulating elements during aperiod of application of each spatial pattern of the second sequence{Q(k)}, e.g., as described above.

In some embodiments, each of the spatial patterns of the second sequence{Q(k)} specifies that the light modulating elements in the identifiedsubset are to nullify their respective portions of the incident lightstream, e.g., as described above.

In some embodiments, the first sequence of spatial patterns {P(k)}includes search patterns representing a partition of the array of lightmodulating elements into a collection of non-overlapping regions. Eachof the search patterns corresponds to a respective one of the regions.(For example, each of the search patterns may specify the ON state forlight modulating elements belonging to the respective region, andspecify the OFF state for light modulating elements outside therespective region.) The action of operating on the first sequence ofsamples {I₁(k)} may include: extracting search samples corresponding tothe search patterns from the first sequence of samples; and determiningone or more of the search samples that exceed a voltage threshold, e.g.,as variously described above.

In some embodiments, the first sequence of spatial patterns {P(k)} mayinclude search patterns, where each of the search patterns correspondsto a respective rectangular subset of the array of light modulatingelements.

In some embodiments, the first sequence of spatial patterns {P(k)}includes a sequence of overlapping angular sector patterns or a sequenceof overlapping concentric annular patterns.

In some embodiments, the first sequence of spatial patterns includes asequence of overlapping parallel band patterns, where the action ofoperating on the first sequence of samples {I₁(k)} includes identifyinga subset of the first sequence of samples that correspond to the bandpatterns and selecting one of the samples from the identified subsetwhose amplitude is maximum. In one embodiment, the search also includesinserting a sequence of rectangle patterns in said first sequence ofspatial patterns {P(k)}, where the sequence of rectangle patterns isrestricted to the band pattern corresponding to the selected samplewhose amplitude is maximum.

In some embodiments, system 2500 may include the analog-to-digitalconverter 140 as described above (or the analog-to-digital converter 640described above). See FIG. 26. The ADC 140 may be configured to acquiresamples (e.g., the samples {I₁(k)} and the samples {I₁(k)}) of theelectrical signal v(t) produced by the light sensing device 130.

In some embodiments, system 2500 may include the control unit 120 asdescribed above. See FIG. 27. Control unit 120 may be configured toperform the function of generating the spatial patterns and/or supplyingthe spatial patterns to the light modulation unit 110. Thus, processor2520 need not generate spatial patterns itself, but may direct thecontrol unit 120 to generate the spatial patterns. For example,processor 2520 may send information to the control unit specifying thekind of spatial patterns (or characterizing parameters of the spatialpatterns) that are to be used in any given context.

Increasing Dynamic Range by Excluding Bright Objects

Compressive sensing devices can apply novel techniques to furtherimprove dynamic range performance, and achieve performance beyond thefull scale performance of an ideal range scaling ADC.

One technique is the removal of intensely bright objects from the imagebefore compressive sensing sensor measurements are made. In conventionalcameras, and especially infrared cameras, the sun can saturate elementsof the focal plane array. When this saturation happens, the signal canbleed into adjacent pixels or even wipe out an entire row or column ofthe image, so that not only the directly-saturated pixels are effected.In a CS device, before starting to read sensor measurements for an imageframe, a number of spatial patterns may be supplied to the DMD todetermine if a hot (intensely bright) spot exists on the micromirrorarray. One approach is a binary hot-spot search where initially thetotal energy on one half of the mirrors is compared to the total energyon the other half of the mirrors. If a significant difference inbrightness exists, a similar binary test can be applied to the brighthalf to further locate the hot spot. This algorithm may be appliediteratively until the location of the hot spot is determined. Once thehot spot is detected and located, the patterns may be adjusted so thatthe mirrors that “see” the hot spot are never pointed at the sensorduring the multiple samples needed to reconstruct the image frame. Afterthe image is algorithmically reconstructed using the multiple samples,the image can be post processed to insert white information where thosehot spot mirrors were located. Eliminating the brightest mirrors allowsthe scale of the range-scaling ADC to be set to provide betterluminosity resolution and to prevent the noise in the bright source fromswamping the signal from all the other mirrors (which would limit thedynamic range of the image on the low end).

A number of search techniques can be employed to find mirrors within ahot region of the DMD array. The binary search algorithm mentioned aboveis just one example of such a search.

Increasing Dynamic Range by Partially Excluding Bright Objects

In a variation on the pixel exclusion algorithm above, another approachis to dither the micro-mirror associated with a discovered brightobject. The rate of on-states for a specific micro-mirror (where ‘on’means that the image light is reflected to the detector) would bedecreased from the normal compressive-sensing light modulation rate by ascaling factor. The result would be that a mirror would have a dutycycle lower than the nominal 50%. For example, with a 60% scalingfactor, the mirror would nominally be in an on-state for 30% of thetime.

Compressive Imaging System 2800

In one set of embodiments, a compressive imaging system 2800 may beconfigured as shown in FIG. 28. The compressive imaging (CI) system mayinclude an optical system 2810, a spatial light modulator 2815, a set2820 of one or more photodetectors, a set 2825 of one or more amplifiers(i.e., one amplifier per detector), a set 2830 of analog-to-digitalconverters (one ADC per detector), and a processing element 2840.

The optical system 2810 focuses an incident light stream onto thespatial light modulator, e.g., as variously described above. See thediscussion above regarding optical subsystem 105. The incident lightstream carries an image (or a spectral ensemble of images) that is to becaptured by the CI system in compressed form.

The spatial light modulator 2815 modulates the incident light streamwith a sequence of spatial patterns to obtain a modulated light stream,e.g., as variously described above.

Each of the detectors 2820 generates a corresponding electrical signalthat represents the intensity of a corresponding portion of themodulated light stream, e.g., a spatial portion or a spectral portion ofthe modulated light stream.

Each of the amplifiers 2825 (e.g., transimpedance amplifiers) amplifiesthe corresponding detector signal to produce a corresponding amplifiedsignal.

Each of the ADCs 2830 acquires samples of the corresponding amplifiedsignal.

The processing element 2840 may operate on the sample sets obtained bythe respective ADCs to construct respective images. The images mayrepresent spatial portions or spectral slices of the incident lightstream. Alternatively, or additionally, the processing element may sendthe sample sets to a remote system for image construction.

The processing element 2840 may include one or more microprocessorsconfigured to execute program instructions stored in a memory medium.

The processing element 2840 may be configured to control one or moreother elements of the CI system. For example, in one embodiment, theprocessing element may be configured to control the spatial lightmodulator, the transimpedance amplifiers and the ADCs.

The processing element 2840 may be configured to perform any subset ofthe above-described methods on any or all of the detector channels. Eachof the detector channels of system 2800 may include any subset of theembodiments, features, and elements described above. (A detector channelmay include a corresponding detector, amplifier and ADC.) For example,each detector channel may include any of the above-described mechanismsfor dynamic range optimization and/or any of the above-describedmechanisms for the identification and attenuation of hot spots in theincident light field.

Compressive Imaging System 2900

In one set of embodiments, a compressive imaging system 2900 may beconfigured as shown in FIG. 29. The compressive imaging system includesthe light modulation unit 110 as variously described above, and alsoincludes optical subsystem 2910, a set of L light sensing devices LSD₁through LSD_(L), and a set of L signal acquisition channels C₁ throughC_(L), where L in a positive integer.

The light modulation unit 110 receives an incident light stream andmodulates the incident light stream with a sequence of spatial patternsto obtain a modulated light stream MLS, e.g., as variously describedabove.

The optical subsystem 2910 delivers portions (e.g., spatial portions orspectral portions) of the modulated light stream to corresponding onesof the light sensing devices LSD₁ through LDS_(L).

For information on various mechanisms for delivering spatial subsets ofthe modulated light stream to respective light sensing devices, pleasesee U.S. patent application Ser. No. 13/197,304, filed on Aug. 3, 2011,titled “Decreasing Image Acquisition Time for Compressive ImagingDevices”, invented by Woods et al., which is hereby incorporated byreference in its entirety as though fully and completely set forthherein.

In some embodiments, the optical subsystem 2910 includes one or morelenses and/or one or more mirrors arranged so as to deliver spatialportions of the modulated light stream onto respective ones of the lightsensing devices. For example, in one embodiment, the optical subsystem2910 includes a lens whose object plane is the plane of the array oflight modulating elements and whose image plane is a plane in which thelight sensing devices are arranged. The light sensing devices may bearranged in an array.

In some embodiments, optical subsystem 2910 is configured to separatethe modulated light stream into spectral components and deliver thespectral components onto respective ones of the light sensing devices.For example, optical subsystem 2910 may include a grating, aspectrometer, or a tunable filter such as a Fabry-Perot Interferometerto achieve the spectral separation.

Each light sensing device LSD_(j) generates a corresponding electricalsignal v_(j)(t) that represents intensity of the corresponding portionMLS_(j) of the modulated light stream.

Each signal acquisition channel C, acquires a corresponding sequence ofsamples {V_(j)(k)} of the corresponding electrical signal v_(j)(t). Eachsignal acquisition channel may include a corresponding amplifier (e.g.,a TIA) and a corresponding A/D converter.

The sample sequence {V_(j)(k)} obtained by each signal acquisitionchannel C_(j) may be used to construct a corresponding sub-image whichrepresents a spatial portion or a spectral slice of the incident lightstream. The number of samples m in each sample sequence {V_(j)(k)} maybe less (typically much less than) the number of pixels in thecorresponding sub-image. Thus, each signal acquisition channel C_(j) mayoperate as a compressive sensing camera for a spatial portion orspectral portion of the incident light.

Each of the signal acquisition channels may include any subset of theembodiments, features, and elements described above.

System 2900 may also include the optical subsystem 2010 (or the opticalsubsystem 105) for focusing the incident light stream onto the lightmodulation unit 110.

Any of the various embodiments described herein may be combined to formcomposite embodiments. Furthermore, any of the various embodimentsdescribed in U.S. Provisional Application No. 61/372,826 and in U.S.patent application Ser. Nos. 13/193,553, 13/193,556 and 13/197,304 maybe combined with any of the various embodiments described herein to formcomposite embodiments.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

What is claimed is:
 1. A system comprising: a light sensing deviceconfigured to receive a modulated light stream and to generate anelectrical signal that represents intensity of the modulated lightstream as a function of time, wherein the modulated light stream isproduced by modulating an incident stream of light with a sequence ofspatial patterns; an amplifier configured to amplify an input signal inorder to obtain an amplified signal, wherein a gain of the amplifier iscontrollable; an analog-to-digital converter (ADC) configured to samplethe amplified signal in order to obtain a sequence of samples; aswitching subsystem configured to controllably select one of thefollowing to be the input signal to the amplifier: (a) the electricalsignal from the light sensing device, or (b) a difference signalrepresenting a difference between the electrical signal and anadjustable baseline voltage.
 2. The system of claim 1, furthercomprising a processing unit configured to: direct the switchingsubsystem to select (b) as the input signal to the amplifier, whereinthe adjustable baseline voltage is set to be approximately equal to anaverage value of the electrical signal; and set the gain of theamplifier based on an estimate of a standard deviation of the electricalsignal.
 3. The system of claim 2, wherein the said modulating of theincident light stream is performed by a light modulation unit having Nlight modulating elements, wherein the estimate of the standarddeviation is a scalar multiple of 1/Sqrt(N).
 4. The system of claim 1,further comprising: a digital-to-analog converter (DAC) configured togenerate the adjustable baseline voltage based on a digital input word.5. The system of claim 4, further comprising a processing unitconfigured to: set the digital input word of the DAC based on anestimate of an average value of the electrical signal; direct theswitching subsystem to select (b) as the input signal to the amplifier;and set the gain of the amplifier based on a standard deviation of theelectrical signal.
 6. The system of claim 4, further comprising: a sumcircuit configured to determine the difference signal by summing theelectrical signal and the negative of the adjustable baseline voltage.7. The system of claim 1, further comprising a processing unitconfigured to: set a gain of the amplifier to a first value; direct theswitching subsystem to select (a) as the input signal to the amplifier;and determine an estimate for an average value of the electrical signalbased on one or more of the samples of said sequence of samples.
 8. Thesystem of claim 7, wherein the one or more samples correspond to one ormore calibration patterns included in the sequence of spatial patterns.9. The system of claim 8, wherein the one or more calibration patternsinclude a maximal light transfer pattern and a minimal light transferpattern.
 10. The system of claim 8, wherein the one or more calibrationpatterns include at least one 50% transfer pattern, wherein the 50%transfer pattern is configured so that 50% of the power in the incidentlight stream passes on average into the modulated light stream.
 11. Thesystem of claim 8, wherein the one or more calibration patterns includeone or more checkerboard patterns.
 12. The system of claim 1, whereinthe light sensing device includes a transimpedance amplifier (TIA),wherein the electrical signal is produced by the TIA.
 13. The system ofclaim 1, wherein the spatial patterns are pseudo-random spatialpatterns.
 14. The system of claim 1, further comprising: a filterconfigured to low-pass filter the electrical signal in order to generatethe adjustable baseline voltage.
 15. The system of claim 1, wherein thesequence of samples includes a set of samples corresponding to M of thespatial patterns, wherein the set of samples is usable to construct anN-pixel image representing the incident light stream, wherein M issmaller than N.
 16. A method comprising: amplifying an electrical signalwith a first gain in order to obtain a first amplified signal, whereinsaid amplifying is performed by an amplifier, wherein the electricalsignal represents intensity of a modulated light stream as a function oftime, wherein the modulated light stream is produced by modulating anincident stream of light with a sequence of spatial patterns; acquiringone or more samples of the first amplified signal corresponding to oneor more calibration patterns included in the sequence of spatialpatterns; determining an estimate of an average value of the electricalsignal based on the one or more samples; amplifying a difference betweenthe electrical signal and an adjustable baseline voltage with a secondgain in order to obtain a second amplified signal, wherein theadjustable baseline voltage is approximately equal to the average valueestimate, wherein the second gain is larger than the first gain;acquiring samples of the second amplified signal corresponding tomeasurement patterns included in the sequence of spatial patterns. 17.The method of claim 16, wherein the measurement patterns arepseudo-random patterns.
 18. The method of claim 16, wherein the samplesof the second amplified signal include a set of samples corresponding toM of the measurement patterns, wherein the set of samples is usable toconstruct an N-pixel image representing the incident light stream, whereM is smaller than N.
 19. The method of claim 16, wherein the second gainis based on an estimate of a standard deviation of the electricalsignal.
 20. The method of claim 19, wherein said modulating the incidentlight stream is performed by a light modulation unit having N lightmodulating elements, wherein the standard deviation is determinedaccording the expression AVG/√{square root over (N)}, wherein AVG is theestimate of the average value of the electrical signal.
 21. The methodof claim 16, further comprising: low-pass filtering the electricalsignal to generate the adjustable baseline voltage.
 22. The method ofclaim 16, wherein the one or more calibration patterns include a maximallight transfer pattern and a minimal light transfer pattern.
 23. Asystem comprising: a plurality light sensing devices configured toreceive respective portions of a modulated light stream and generaterespective electrical signals, wherein each of the electrical signalsrepresents intensity of the respective portion of the modulated lightstream as a function of time, wherein the modulated light stream isproduced by modulating an incident stream of light with a sequence ofspatial patterns; a plurality of amplifiers configured to amplifyrespective input signals in order to obtain respective amplifiedsignals, wherein a gain of each amplifier is controllable; a pluralityof analog-to-digital converters (ADCs), wherein each ADC is configuredto sample the respective amplified signal in order to obtain arespective sequence of samples; a plurality of switching subsystems,wherein each of the switching subsystems corresponds to a respective oneof the amplifiers and a respective one of the ADCs, wherein each of theswitching subsystems is configured to controllably select one of thefollowing to be the input signal to the respective amplifier: (a) theelectrical signal from the respective light sensing device, or (b) adifference signal representing a difference between the respectiveelectrical signal and a respective adjustable baseline voltage.